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  • 1. Aldongarov, Anuar
    et al.
    Irgibaeva, Irina
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Theoretical study on passivation of small CdS clusters2014Inngår i: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 112, nr 5-6, s. 674-682Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We use density functional theory quantum chemical calculations to model optical properties of small cadmium sulphide clusters in order to explore the formation of trap states and the possibility to achieve surface passivation. The addition of capping oxygen ions on the surface of the CdS quantum dots is found to passivate the single-bonded Cd atoms. Added Cu ions resulted in the formation of deep trap states for certain locations of the copper impurity, while removal of single-bonded Cd atoms by adding S or SH groups leads to a band gap increase.

  • 2. Alfredsson, M.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    OH frequency calculations for the hydroxylated MgO(001) surface2002Inngår i: Molecular Simulation, ISSN 0892-7022, E-ISSN 1029-0435, Vol. 28, nr 07-jun, s. 663-681Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have performed periodic Hartree-Fock calculations for OH groups adsorbed on the MgO(001) surface considering different surface coverages. Six types of OH groups are discussed: OH-, OH, H+, H and hydrogen-bonded OH and H. It is found that when both OH and H are present on the surface, the two groups are best described as OH-. We suggest that the highest-frequency fundamental band (similar to3750 cm(-1) in the experimental OH spectrum) is assigned to OH- groups adsorbed on top of Mg2+, while H+ adsorbed on top of O2- give rise to the broader band at similar to3550 cm(-1).

  • 3. Alfredsson, M.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Dovesi, R.
    Periodic ab initio calculations of the spontaneous polarisation in ferroelectric NaNO22002Inngår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 4, nr 17, s. 4204-4211Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present periodic ab initio calculations for ferroelectric NaNO2. The spontaneous polarisation (P-s) has been calculated with three different models based on: (i) point charges; (ii) a multipolar expansion of the charge ;distribution; and (iii) the Berry phase approach. Both the Hartree-Fock and LDA Hamiltonians were employed. Within the Hartree-Fock scheme, at the optimised geometry, we obtain P-s values of 20.3, 13.0 and 16.4 muC cm(-2) with the three models, compared to the experimental value of similar to11.9 muC m(-2). The Berry-phase approach at the Hartree-Fock level gives a value very close to experiment ( namely 12 muC cm(-2)) when the experimental structure is used. At the optimised LDA structure, the LDA P-s values are 16.8, 10.0 and 16.9 muC cm(-2) with models (i)-(iii). The optimised lattice parameters at the Hartree-Fock level are slightly shorter (between 0 and 4%) than those determined experimentally from X-ray-diffraction, while the lattice parameters of the LDA-optimised structure are up to 10% smaller than the experiment. The calculated lattice energies are 679 and 964 kJ mol(-1) at the HF and LDA levels, compared with 729 kJ mol(-1) from the experiment. Charge densities, Mulliken charges and dipole moments are discussed. Finally, we have studied the mechanism for the phase transformation in-between the ferroelectric and paraelectric crystal structures; a rotation of the NO2 group around the c-axis gives the lowest energy barrier.

  • 4. Amira, S.
    et al.
    Spangberg, D.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Derivation and evaluation of a flexible SPC model for liquid water2004Inngår i: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 303, nr 3, s. 327-334Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Molecular dynamics simulations of a new flexible water model are presented. The potential function is based on the simple point charge (SPC) model combined with an accurate experimental quartic intramolecular potential (CCL). This potential not only reproduces the key structural, dynamical and thermodynamical properties of liquid water, but also generates the correct values for both the absolute O-H vibrational frequency and the gas-to-liquid frequency shift. Thus, the model yields a value of ca. 3420 cm(-1) for the peak maximum of the anharmonic OH stretching band for liquid water (experiment 3400 cm(-1)) and a corresponding gas-to-liquid downshift of -300 cm(-1) (experiment -310 cm(-1)). The paper also emphasizes the importance of parameterizing correctly both the harmonic and anharmonic force constants in the gas-phase when developing flexible water models to be used in the liquid state.

  • 5. Amira, S.
    et al.
    Spangberg, D.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Distorted five-fold coordination of Cu2+ (aq) from a Car-Parrinello molecular dynamics simulation2005Inngår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 7, nr 15, s. 2874-2880Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The solvation shell structure and dynamics of a single Cu2+ ion in a periodic box with 32 water molecules under ambient conditions has been investigated using Car-Parrinello molecular dynamics simulations in a time-window of 18 ps. Five-fold coordination with four equidistant equatorial water molecules at 2.00 angstrom and one axial water molecule at 2.45 angstrom from the Cu2+ ion is found. A hole without water molecules is found on the opposite side of the axial water. The ion-water bonding character for the equatorial water molecules is different from that of the axial water molecules, as shown by a localized orbital analysis of the electronic structure. Moreover, the calculated OD stretching vibrational band for the equatorial water molecules lies ca. 175 cm(-1) below the axial-water band, in good agreement with experimental data. The equatorial-water band lies below, and the axial-water band above, the pure liquid D2O band, also in agreement with experimental data.

  • 6. Amira, S.
    et al.
    Spangberg, D.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    OD vibrations and hydration structure in an Al3+(aq) solution from a Car-Parrinello molecular-dynamics simulation2006Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 124, nr 10Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The optimized geometry, energetics, and vibrational properties of Al(D2O)(n)(3+) clusters, with n=1,2,4, and 6, have been studied using plane waves, different local basis sets, different methodologies [density-functional theory, MP2, CCSD(T)], and different functionals (BLYP, PBE). Moreover, Car-Parrinello molecular-dynamics (MD) simulations using the BLYP functional, plane waves, and the Vanderbilt ultrasoft pseudopotentials have been performed for an aqueous Al3+ solution with 1 ion and 32 D2O molecules in a periodic box at room temperature, studied for 10 ps. The cluster calculations were performed to pinpoint possible shortcomings of the electronic structure description used in the Car-Parinello MD (CPMD) simulation. For the clusters, the hydration structure and interaction energies calculated with the 'BLYP/plane-wave' approach agree well with high-level ab initio methods but the exchange-correlation functional introduces errors in the OD stretching frequencies (both in the absolute values and in the ion-induced shifts). For the aqueous solution, the CPMD simulation yields structural properties in good agreement with experimental data. The CPMD-simulated OD stretching vibrational band for the first-shell water molecules around Al3+ is strongly downshifted by the influence of the ion and is compared with experimental data from the literature. To make such a comparison meaningful, the influences of a number of systematic effects have been addressed, such as the exchange-correlation functional, the fictitious electron mass, anharmonicity effects, and the small box size in the simulation. Each of these factors (except the last one) is found to affect the OD frequency by 100 cm(-1) or more. The final corrected frequencies agree with experiment within similar to 30 cm(-1) for bulk water but are too little downshifted for the first-shell Al3+(aq) water molecules (by similar to 200 cm(-1)).

  • 7. Amira, S.
    et al.
    Spangberg, D.
    Probst, M.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Molecular dynamics simulation of Fe2+(aq) and Fe3+(aq)2004Inngår i: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 108, nr 1, s. 496-502Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Molecular dynamics simulations of single-ion Fe2+(aq) and Fe3+(aq) solutions have been performed with two rigid-water models (SPC and SPC/E) and a newly constructed SPC-based flexible-water model (SPC+CCL). The SPC+CCL water model in combination with effective Fe2+ and Fe3+ ion-water potentials manages to reproduce many experimental structural and dynamical properties of the solutions. Special attention is given to the large ion-induced frequency shifts of the OH stretching bands, which are also well reproduced by the SPC+CCL model.

  • 8. Amira, S.
    et al.
    Spangberg, D.
    Zelin, V.
    Probst, M.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Car-Parrinello molecular dynamics simulation of Fe3+(aq)2005Inngår i: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 109, nr 29, s. 14235-14242Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The optimized geometry and energetic properties of Fe(D2O)(n)(3+) clusters, with n = 4 and 6, have been studied with density-functional theory calculations and the BLYP functional, and the hydration of a single Fell ion in a periodic box with 32 water molecules at room temperature has been studied with Car-Parrinello molecular dynamics and the same functional. We have compared the results from the CPMD simulation with classical MD simulations, using a flexible SPC-based water model and the same number of water molecules, to evaluate the relative strengths and weaknesses of the two MD methods. The classical MD simulations and the CPMD simulations both give Fe-water distances in good agreement with experiment, but for the intramolecular vibrations, the classical MD yields considerably better absolute frequencies and ion-induced frequency shifts. On the other hand, the CPMD method performs considerably better than the classical MD in describing the intramolecular geometry of the water molecule in the first hydration shell and the average first shell(...)second shell hydrogen-bond distance. Differences between the two methods are also found with respect to the second-shell water orientations. The effect of the small box size (32 vs 512 water molecules) was evaluated by comparing results from classical simulations using different box sizes; non-negligible effects are found for the ion-water distance and the tilt angles of the water molecules in the second hydration shell and for the O-D stretching vibrational frequencies of the water molecules in the first hydration shell.

  • 9. Baudin, M.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Metal oxide surface dynamics from molecular dynamics simulations: the alpha-Al2O3(0001) surface2001Inngår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 474, nr 03-jan, s. 107-113Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Constant-stress, constant-temperature (10, 300 and 700 K) molecular dynamics simulations were carried out with shell-model potentials for an infinite crystalline A-terminated alpha -Al2O3(000 1) slab of similar to 25 Angstrom thickness. The surface undergoes considerable relaxation at 10 K, but exhibits ordered surface structures at all three temperatures. The relaxation causes the (0 0 0 1) surface at 10 K to appear oxygen-terminated. The ionic motion within the central and surface regions of the slab system has been analysed in terms of mean-square displacements. At room temperature the (u(2))(surface)/(u(2))(bulk) ratio for the Al ions is approximate to2.5 and when only the out-of-plane surface motion is considered the ratio is as large as approximate to3.5. The O ion motion at the surface is slightly smaller than that of the cations.

  • 10. Baudin, M.
    et al.
    Wojcik, M.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Dynamics, structure and energetics of the (111), (011) and (001) surfaces of ceria2000Inngår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 468, nr 03-jan, s. 51-61Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Molecular dynamics simulations for 20-30 Angstrom thick CeO2 slabs with 2-D periodicity are presented. The three low-index surfaces investigated are(lll), (011) and (001). The simulations were performed within a constant-temperature-constant-pressure ensemble and used the shell-model to describe polarizability. All simulation runs were performed at atmospheric pressure and in the temperature range 10-1100 K. For all three surfaces at both 300 and 1100 K, we find that the surface m.s. displacements are generally larger for the oxide ions than for the cations and that the out-of-plane surface motion is usually larger than the in-plane surface motion. At room temperature, the oxygen m.s. displacements at the (Ill)surface are a factor 1.2 larger than in the bulk, a factor 1.6 for the (011) surface and approximately five times larger at the metastable (001) surface compared to the bulk. The effect of the presence of a surface on the ion dynamics [and on the structure for (011)] persists all the way to the slab centres. even for these rather thick slabs. Our simulations for the polar (001) surface demonstrate that the relative stabilities of different faces and surface terminations can change with temperature, and that it may not always be meaningful to consider one specific user-prepared termination as superior to the others.

  • 11. Baudin, M.
    et al.
    Wojcik, M.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Molecular dynamics simulations of an Al2O3(0001 +/-, 0-10(II))/CeO2 (011 +/-,01-1(II)) interface system2001Inngår i: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 401, nr 02-jan, s. 159-164Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Constant stress, constant temperature (10 K, 300 K) molecular dynamics simulations were carried out with shell-model potentials for an infinite composite ceria-alumina slab with two free surfaces [alpha -Al2O3 (0001) and CeO2(011) and their opposite counterparts]. The interface introduces considerable structural and dynamical changes, both at the slab surfaces and in the center of the slab. Structurally, both oxide surfaces become effectively oxygen-terminated and the surface structures become disordered close to the interface. Dynamically, in the region near the 'alumina surface/ceria surface/alumina-ceria interface' 3-phase junction the ionic motion is considerably enhanced. Thus, in the interface region, the ionic mean-square displacements increase 2-3 times compared to the pure slabs. Moreover, the ions at the interface participate in a new kind of motion, not present in the pure oxide slabs: large occasional, but frequently reoccurring, back-and-forth ionic motions take place with square-amplitudes as large as similar to0.70 Angstrom (2).

  • 12. Baudin, M.
    et al.
    Wojcik, M.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Palmqvist, A. E. C.
    Muhammed, Mamoun
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    MD simulations of a doped ceria surface - very large surface ion motion2001Inngår i: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 335, nr 06-maj, s. 517-523Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Mean-square displacements (MSDs) and individual-ion square-displacements (ISDs) for the different constituents in Ca-doped CeO2(0 1 1) slabs at 300 K have been studied as a function of depth from the surface. Constant pressure-constant temperature MD simulations were used. The MSDs are 2-3 times larger at the surface than in the bulk, but ISDs as large as ca. 150 times the surface MSD value were observed during short-time periods for anions next to an anion vacancy at the surface. The chemical implications of this kind of motion are important, since transient structural distortions of this magnitude will lead to large electron re-distributions.

  • 13. Castleton, C. W. M.
    et al.
    Kullgren, J.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Tuning LDA+U for electron localization and structure at oxygen vacancies in ceria2007Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 127, nr 24Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We examine the real space structure and the electronic structure (particularly Ce4f electron localization) of oxygen vacancies in CeO2 (ceria) as a function of U in density functional theory studies with the rotationally invariant forms of the LDA+U and GGA+U functionals. The four nearest neighbor Ce ions always relax outwards, with those not carrying localized Ce4f charge moving furthest. Several quantification schemes show that the charge starts to become localized at U approximate to 3 eV and that the degree of localization reaches a maximum at similar to 6 eV for LDA+U or at similar to 5.5 eV for GGA+U. For higher U it decreases rapidly as charge is transferred onto second neighbor O ions and beyond. The localization is never into atomic corelike states; at maximum localization about 80-90% of the Ce4f charge is located on the two nearest neighboring Ce ions. However, if we look at the total atomic charge we find that the two ions only make a net gain of (0.2-0.4)e each, so localization is actually very incomplete, with localization of Ce4f electrons coming at the expense of moving other electrons off the Ce ions. We have also revisited some properties of defect-free ceria and find that with LDA+U the crystal structure is actually best described with U=3-4 eV, while the experimental band structure is obtained with U=7-8 eV. (For GGA+U the lattice parameters worsen for U > 0 eV, but the band structure is similar to LDA+U.) The best overall choice is U approximate to 6 eV with LDA+U and approximate to 5.5 eV for GGA+U, since the localization is most important, but a consistent choice for both CeO2 and Ce2O3, with and without vacancies, is hard to find.

  • 14. Castleton, Christopher W. M.
    et al.
    Nokbin, Somkiat
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Correlations between magnetic properties and bond formation in Rh-MgO(001)2007Inngår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 601, nr 5, s. 1218-1230Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present the results of first principles calculations for the magnetism of Rh adlayers on MgO(001), at three different adsorption sites and three different coverages, corresponding to 1, 1/2 and 1/8 monolayers. Finite magnetization is found in all cases except that of one Rh monolayer above the oxygen site, which is also the most stable. We examine how the magnetization changes as a function of the Rh-surface distance and relate this to changes in the real-space charge density and in the density of states (DOS) as the Rh adlayer interacts with the surface. We find that increasing either the Rh-Rh interaction strength or the Rh-surface interaction strength leads to reduced magnetization, while increasing the former drives a crossover from localized (atomic) to itinerant magnetism. Neither the magnetic transition itself, nor the localized-to-itinerant magnetism crossover, is found to be directly related to the formation of Rh-surface bonds. From a practical point of view, we predict that magnetism in the Rh-MgO(001) system is most likely to be found experimentally at reduced coverages and at low temperatures.

  • 15.
    Duan, Sai
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Xu, Xin
    Luo, Yi
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Tian, Zhong-Qun
    Thermal effects on electronic properties of CO/Pt(111) in water2013Inngår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 15, nr 32, s. 13619-13627Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Structure and adsorption energy of carbon monoxide molecules adsorbed on the Pt(111) surfaces with various CO coverages in water as well as work function of the whole systems at room temperature of 298 K were studied by means of a hybrid method that combines classical molecular dynamics and density functional theory. We found that when the coverage of CO is around half monolayer, i.e. 50%, there is no obvious peak of the oxygen density profile appearing in the first water layer. This result reveals that, in this case, the external force applied to water molecules from the CO/Pt(111) surface almost vanishes as a result of the competitive adsorption between CO and water molecules on the Pt(111) surface. This coverage is also the critical point of the wetting/non-wetting conditions for the CO/Pt(111) surface. Averaged work function and adsorption energy from current simulations are consistent with those of previous studies, which show that thermal average is required for direct comparisons between theoretical predictions and experimental measurements. Meanwhile, the statistical behaviors of work function and adsorption energy at room temperature have also been calculated. The standard errors of the calculated work function for the water-CO/Pt(111) interfaces are around 0.6 eV at all CO coverages, while the standard error decreases from 1.29 to 0.05 eV as the CO coverage increases from 4% to 100% for the calculated adsorption energy. Moreover, the critical points for these electronic properties are the same as those for the wetting/non-wetting conditions. These findings provide a better understanding about the interfacial structure under specific adsorption conditions, which can have important applications on the structure of electric double layers and therefore offer a useful perspective for the design of the electrochemical catalysts.

  • 16. Dungsrikaew, V.
    et al.
    Limtrakul, J.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Probst, M.
    Comparison of methods for point-charge representation of electrostatic fields2004Inngår i: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 96, nr 1, s. 17-22Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The calculation of the electrostatic potential resulting from an infinite or extended array of charges in the interior of a region of interest is a frequent task in computational chemistry. In case of a periodic potential this can, for example, be done by Ewald summation or by multipole methods. An important alternative are those methods where arrays of auxiliary point charges are optimized with respect to charge and/or position to reproduce the original electrostatic potential. In the literature different variations are reported. We compare the performance of some of these with respect to their ability to reproduce the original potential and the computational effort required. Between (1) surface charges determined by the conductor-boundary condition, (2) optimized surface charges, and (3) surface charges floating on the surface we find that (2) offers good quality with small computational costs involved.

  • 17. Gotte, A.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Baudin, M.
    Molecular dynamics simulations of reduced CeO2: bulk and surfaces2004Inngår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 552, nr 03-jan, s. 273-280Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Results from molecular dynamics simulations for bulk and the (0 1 1) surface of reduced and unreduced CeO2 at 300 K are reported. The presence of vacancies in reduced ceria is found to give rise to many different local structural arrangements, and to a significant broadening of the peaks in the pair-distribution function. This broadening is mainly due to the appearance of these new structural arrangements, and it is not due to the increased ionic motions in the reduced systems.

  • 18. Gotte, A.
    et al.
    Spangberg, D.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Baudin, M.
    Molecular dynamics study of oxygen self-diffusion in reduced CeO22007Inngår i: Solid State Ionics, ISSN 0167-2738, E-ISSN 1872-7689, Vol. 178, nr 25-26, s. 1421-1427Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The oxygen self-diffusion in partially reduced CeO2 has been investigated by large-scale Molecular Dynamics simulations, in the temperature range between 800 and 2200 K. Simulation boxes with similar to 4100 and similar to 33,000 ions were investigated for randomly distributed oxygen vacancies and Ce3+ ions. Our calculated self-diffusion coefficients vary between 10(-8) and 10(-6) cm(2)/s in the temperature range studied. The activation energy and D-0 values are also reported. The oxygen diffusion mechanism has also been analyzed: only a (< 100 > vacancy mechanism is observed.

  • 19. Guo, J. H.
    et al.
    Augustsson, A.
    Kashtanov, Stepan
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Spångberg, D.
    Nordgren, J.
    Hermansson, Kersti
    Luo, Yi
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Augustsson, A.
    The interaction of cations and liquid water studied by resonant soft-X-ray absorption and emission spectroscopy2005Inngår i: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 144, s. 287-290Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report the soft-X-ray absorption and emission studies of NaCl, MgCl2, and AlCl3 in water solutions. The influences of cations on the water molecular structure can be seen as the absorption threshold edge shifted to high energy in the X-ray absorption spectra; the mixing of molecular orbital in 3a(1) symmetry is reinforced as the intensity of 3a(1) is further reduced; and the 1b(1)-emission peak shows the broadening and shift differently for Na+, Mg2+, and Al3+ water solutions, which indicates that the charge difference of the cations may not be the only playing role being responsible to the interactions between the cations and water molecules.

  • 20. Hellström, Matti
    et al.
    Spångberg, Daniel
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Broqvist, Peter
    Cu dimer formation mechanism on the ZnO(10(1)over-bar0) surface2012Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, nr 23, s. 235302-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The formation of Cu dimers on the ZnO(10 (1) over bar0) surface has been studied using hybrid density functional theory. Depending on the adsorption site, Cu atoms are found to adsorb with either oxidation state 0 or +1. In the latter case, the Cu atom has donated an electron to the ZnO conduction band. The two modes of adsorption display similar stability at low coverages, while at higher coverages the neutral species is more stable. Single Cu atoms diffuse across the ZnO(10 (1) over bar0) surface with small barriers of migration (0.3-0.4 eV) along ZnO[1 (2) over bar 10], repeatedly switching their oxidation states, while the barrier along ZnO[0001] is significantly higher (>1.5 eV). The formation of a Cu dimer from two adsorbed Cu atoms is energetically favorable with two competing structures of similar stability, both being charge neutral. The minimum energy paths for Cu atom diffusion and dimer formation are characterized by at least one of the two Cu atoms being in oxidation state 0.

  • 21.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Blue-shifting hydrogen bonds2002Inngår i: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 106, nr 18, s. 4695-4702Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper we put forward the idea that the various improper, blue-shifting hydrogen bond systems discussed in the literature are all of essentially the same nature and occur because of three necessary circumstances: (i) the presence of a negative dipole moment derivative, dmu(0)/dr(XH), for the isolated H-bond donor molecule; (ii) the interaction between such a molecule and any electron density concentration on the H-bond acceptor ( pi-system density, lone-pair density, ionic charge,...) which at large intermolecular distances gives rise to a field-dominated, modest vibrational blue shift; (iii) an additional blue shift due to electronic exchange overlap. The negative dipole moment derivative is a necessary but not sufficient condition for the formation of a blue-shifted H-bond: thus, the blue-shifting CH4, F3CH, and Cl3CH molecules and their relatives can also give rise to normal, red-shifted H-bonds. This is a logical extension of the blue-shifting property and occurs when the electric field from the acceptor is sufficiently strong at the intermolecular equilibrium distance (e.g., for F- and Cl- acceptors).

  • 22. Hermansson, Kersti
    Host-guest interactions in an organic crystal: beta-hydroquinone clathrate with Ne and HF guests2000Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 112, nr 2, s. 835-840Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Host-guest, guest-guest, and host-host interactions in the beta-hydroquinone clathrate crystal have been calculated from periodic and cluster-type ab initio calculations. The potential energy surfaces for the positions of the enclosed guest molecules in the cage have also been studied. The host framework remains virtually unchanged upon guest enclosure. The inclusion energy for the polar HF molecule is about -7 to -9 kcal/mol, in good agreement with our extrapolation from existing calorimetric data for HBr and HCl, and about -1 to -2 kcal/mol for Ne. The guest-guest interaction is small.

  • 23.
    Hermansson, Kersti
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Probst, Michael M.
    Gajewski, Grzegorz
    Mitev, Pavlin
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Anharmonic OH vibrations in Mg(OH)(2) (brucite): Two-dimensional calculations and crystal-induced blueshift2009Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, nr 24Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A two-dimensional quantum-mechanical vibrational model has been used to calculate the anharmonic OH vibrational frequencies in the layered Mg(OH)(2) (brucite) crystal. The underlying potential energy surface was generated by density functional theory (DFT) calculations. The resulting OH frequencies are upshifted (blueshifted) by about +75 cm(-1) with respect to the gas-phase OH frequency (+120 cm(-1) in experiments; the discrepancy is mainly due to inadequacies in the DFT and pseudopotential models). The Raman-IR split is about 50 cm(-1), both in the calculations and in experiments. We find that the blueshift phenomenon in brucite can qualitatively be explained by a parabolalike "OH frequency versus electric field" correlation curve pertaining to an OH- ion exposed to an electric field. We also find that it is primarily the neighbors within the Mg(OH)(2) layer that induce the blueshift while the interlayer interaction gives a smaller (and redshifting) contribution.

  • 24. Herschend, B.
    et al.
    Baudin, M.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    A combined molecular dynamics plus quantum mechanics method for investigation of dynamic effects on local surface structures2004Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 120, nr 10, s. 4939-4948Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A combined molecular dynamics (MD)+quantum mechanics (QM) method for studying processes on ionic surfaces is presented. Through the combination of classical MD and ab initio embedded-cluster calculations, this method allows the modeling of surface processes involving both the structural and dynamic features of the substrate, even for large-scale systems. The embedding approach used to link the information from the MD simulation to the cluster calculation is presented, and rigorous tests have been carried out to ensure the feasibility of the method. The electrostatic potential and electron density resulting from our embedded-cluster model have been compared with periodic slab results, and confirm the satisfying quality of our embedding scheme as well as the importance of applying embedding in our combined MD+QM approach. We show that a highly accurate representation of the Madelung potential becomes a prerequisite when the embedded-cluster approach is applied to temperature-distorted surface snapshots from the MD simulation.

  • 25. Herschend, B.
    et al.
    Baudin, M.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Electronic structure of the CeO2(110) surface oxygen vacancy2005Inngår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 599, nr 03-jan, s. 173-186Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The oxygen vacancy formation on the CeO2(110) surface has been studied by ab initio electronic structure calculations. Embedded-cluster calculations with explicit treatment of the electron correlation from Moller-Plesset perturbation theory (MP2) provide an alternative description of the surface O vacancy compared to previously reported periodic density functional theory (DFT) calculations. The electronic structure at the MP2 level shows a complete localization of the excess electrons on the two surface Ce ions neighboring the vacancy, contrary to the delocalized description seen in the periodic DFT calculations for the CeO2(110) surface (but more in line with DFT+U results recently reported for the partially reduced CeO2 bulk and (001)-surface). Our calculations predict a vacancy formation energy (3.1-3.3 eV at the MP2 level including basis set superposition error (BSSE) correction) and a geometric structure in qualitative agreement with the periodic DFT results, where the surface O ion next to the vacancy assumes a bridging position between the reduced Ce ions.

  • 26. Herschend, B.
    et al.
    Baudin, M.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Influence of substrate dynamics on CO-MgO(001) bonding-using molecular dynamics snapshots in quantum-chemical calculations2006Inngår i: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 110, nr 11, s. 5473-5479Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Combined molecular dynamics (MD) and quantum mechanics QM) calculations have been performed for CO adsorbed on MgO(001) at 50 K. The changes in the adsorption energy caused by the surface dynamics have been analyzed, and a clear correlation was found between the dynamic variation of the adsorption energy and the electrostatic field above the adsorption site. By separating the electrostatic contributions arising from the local structure at the adsorption site from those originating from the rest of the slab, a linear expression of these contributions could be fitted which closely reproduces the dynamic changes in the adsorption energy. Using this simple linear expression, the distribution of adsorption energies for CO above the Mg2+ sites on the MgO(001) surface at 50, 80, and 150 K have been predicted.

  • 27. Herschend, B.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Alfredsson, M.
    Zhukovskii, Y. F.
    Kotomin, E. A.
    Jacobs, P. W. M.
    Characterization of the metal-ceramic bonding in the Ag/MgO(001) interface from ab initio calculations2003Inngår i: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 107, nr 43, s. 11893-11899Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The nature of the metal-ceramic interaction in the Ag/MgO(001) interface is studied using periodic Hartree-Fock calculations with density functional theory a posteriori correlation corrections. Different aspects of the Ag-MgO interaction have been studied by analysis of the electronic properties: total and projected density of states, multipole moments, bond population, and the difference electron density. By linking these properties to the adsorption energy and making a comparative analysis for interfaces with different degrees of coverage and different adsorption models (one- and two-sided adsorption), a detailed description of the Ag-MgO has been acquired.

  • 28. Herschend, Bjorn
    et al.
    Baudin, Micael
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    CO adsorption on CeO2(110) using hybrid-DFT embedded-cluster calculations2006Inngår i: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 328, nr 03-jan, s. 345-353Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The adsorption of CO on the CeO2 (110) surface has been studied using hybrid density functional theory (DFT) in the form of the B3LYP functional. In our calculations, both physisorption and chemisorption are observed. In the physisorption case, CO binds perpendicularly to a surface Ce ion. The adsorption energy is estimated to 0.1 eV and the CO stretching vibration becomes slightly blue-shifted (+9 cm(-1)), but the changes to the substrate and adsorbate structure are minimal. Test calculations using Moller-Plesset perturbation theory to the second order (MP2) suggest that dispersive interactions may be important in the case of CO physisorption on ceria. A chemisorption site was also found, where the CO molecule binds to two surface O ions, forming a surface carbonate species. Here, B3LYP predicts an adsorption energy of 2.1 eV and a red-shift of 470 cm(-1) for the CO stretching vibration.

  • 29. Herschend, Bjorn
    et al.
    Baudin, Micael
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Oxygen vacancy formation for transient structures on the CeO2(110) surface at 300 and 750 K2007Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 126, nr 23Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Ab initio embedded-cluster calculations have been performed for the CeO2(110) surface using temperature induced structures from molecular dynamics (MD) snapshots. As a first step towards understanding how temperature induced distortions of the surface structure influence the surface oxygen reactivity, the energy cost of removing an O atom from the surface was calculated for 41 snapshots from the MD simulation at 300 K. The quantum mechanical embedded-cluster calculations show that already at 300 K the dynamics causes significant fluctuations (root mean square of 0.37 eV) in the O vacancy formation energy (E-vac) while the distribution of the two excess electrons associated with the vacancy is virtually unaffected by the surface dynamics and remains localized on the two Ce ions close to the vacancy. It is also found that the quantum mechanical E-vac fluctuations can be reproduced by oxygen vacancy calculations using only the relaxed shell-model force field (FF) itself and the MD geometries. Using the FF as the interaction model, the effect of raising the temperature to 750 K and the effect of doping with Ca were investigated for the oxygen vacancy formation.

  • 30. Hu, Shuanglin
    et al.
    Li, S. -Y
    Ahuja, R.
    Granqvist, C. G.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Niklasson, G. A.
    Scheicher, R. H.
    Optical properties of Mg-doped VO2: Absorption measurements and hybrid functional calculations2012Inngår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 101, nr 20, s. 201902-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Mg-doped VO2 thin films with thermochromic properties were made by reactive DC magnetron co-sputtering onto heated substrates, and spectral absorption was recorded at room temperature in the 0.5 < <(h)over bar>omega < 3.5 eV energy range. Clear evidence was found for a widening of the main band gap from 1.67 to 2.32 eV as the Mg/(V + Mg) atomic ratio went from zero to 0.19, thereby significantly lowering the luminous absorption. This technologically important effect could be reconciled with spin-polarized density functional theory calculations using the Heyd-Scuseria-Ernzerhof [Heyd et al., J. Chem. Phys. 118, 8207 (2003); ibid. 124, 219906 (2006)] hybrid functional. Specifically, the calculated luminous absorptance decreased when the Mg/(V + Mg) ratio was increased from 0.125 to 0.250.

  • 31. Jalilehvand, F.
    et al.
    Spangberg, D.
    Lindqvist-Reis, P.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Persson, I.
    Sandstrom, M.
    Hydration of the calcium ion. An EXAFS, large-angle X-ray scattering, and molecular dynamics simulation study2001Inngår i: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 123, nr 3, s. 431-441Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The structure of the hydrated calcium(II) ion in aqueous solution has been studied by means of extended X-ray absorption fine structure spectroscopy (EXAFS), large-angle X-ray scattering (LAXS), and molecular dynamics (MD) methods. The EXAFS data displayed a broad and asymmetric distribution of the Ca-O bond distances with the centroid at 2.46(2) Angstrom. LAXS studies on four aqueous calcium halide solutions (1.5-2 mol dm(-3)) gave a mean Ca-O bond distance of 2.46(1) Angstrom. This is consistent with a hydration number of 8 determined from correlations between mean distances and coordination numbers from crystal structures. The LAXS studies showed a second coordination sphere with a mean Ca . . .O-II distance of 4.58(5) Angstrom, and for the hydrated halide ions the distances Cl . . .O 3.25(1) Angstrom. Br . . .O 3.36(1) Angstrom, and I . .O 3.61(1) Angstrom were obtained. Molecular dynamics simulations of CaCl2(aq) were performed using three different Ca2+-OH2 pair potentials. The potential from the GROMOS program gave results in agreement with experiments, i.e., a coordination number of 8 and an average Ca-O distance of 2.46 Angstrom, and was used for further comparisons. Theoretical EXAFS oscillations were computed far individual Mn snapshots and showed very large variations, though the simulated average spectrum from 2000 snapshots gave satisfactory agreement with the experimental EXAFS spectra. The effect of thermal motions of the coordinated atoms is inherent in the MD simulation method. Thermal disorder parameters evaluated from simulated spatial atom distribution functions of the oxygen atoms coordinated to the calcium ion were in close agreement with those from the current LAXS and EXAFS analyses. The combined results are consistent with a root-mean-square displacement from the mean Ca-O distance of 0.09(2) Angstrom in aqueous solution at 300 K.

  • 32. Kullgren, Jolla
    et al.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Broqvist, Peter
    Supercharged Low-Temperature Oxygen Storage Capacity of Ceria at the Nanoscale2013Inngår i: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 4, nr 4, s. 604-608Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We provide an explanation for the experimental finding of a dramatically enhanced low-temperature oxygen storage capacity for small ceria nanoparticles. At low temperature, small octahedral ceria nanoparticles will be understoichiometric at both oxidizing and reducing conditions without showing explicit oxygen vacancies. Instead, rather than becoming stoichiometric at oxidizing conditions, such particles are stabilized through oxygen adsorption forming superoxo (O-2(-)) ions and become in this way supercharged with oxygen. The supercharging effect is size-dependent and largest for small nanoparticles where it gives a direct increase in the oxygen storage capacity and simultaneously provides a source of active oxygen species at low temperatures.

  • 33. Kullgren, Jolla
    et al.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Castleton, Christopher
    Many competing ceria (110) oxygen vacancy structures: From small to large supercells2012Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 137, nr 4, s. 044705-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present periodic "DFT+U" studies of single oxygen vacancies on the CeO2(110) surface using a number of different supercells, finding a range of different local minimum structures for the vacancy and its two accompanying Ce(III) ions. We find three different geometrical structures in combination with a variety of different Ce(III) localization patterns, several of which have not been studied before. The desired trapping of electrons was achieved in a two-stage optimization procedure. We find that the surface oxygen nearest to the vacancy either moves within the plane towards the vacancy, or rises out of the surface into either a symmetric or an unsymmetric bridge structure. Results are shown in seven slab geometry supercells, p(2 x 1), p(2 x 2), p(2 x 3), p(3 x 2), p(2 x 4), p(4 x 2), and p(3 x 3), and indicate that the choice of supercell can affect the results qualitatively and quantitatively. An unsymmetric bridge structure with one nearest and one next-nearest neighbour Ce(III) ion (a combination of localizations not previously found) is the ground state in all (but one) of the supercells studied here, and the relative stability of other structures depends strongly on supercell size. Within any one supercell the formation energies of the different vacancy structures differ by up to 0.5 eV, but the same structure can vary by up to similar to 1 eV between supercells. Furthermore, finite size scaling suggests that the remaining errors (compared to still larger supercells) can also be similar to 1 eV for some vacancy structures.

  • 34. Lauenstein, A.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Lindgren, J.
    Probst, M.
    Bopp, P. A.
    Molecular dynamics simulation of an aqueous aluminium(III) chloride solution with three-body interactions2000Inngår i: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 80, nr 05-apr, s. 892-906Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A molecular dynamics simulation of a 0.28 molar aqueous AlCl3 solution was performed with the flexible Bopp, Jancso, and Heinzinger (BJH) model for water and a newly developed three-body potential for Al3+-(H2O)(2) interactions derived from ab initio calculations. The simulations extended over 25 ps at an average temperature of 300 K. The structural properties of the solution are discussed on the basis of radial distribution functions and the orientation of the water molecules. Dynamical properties and OH frequency shifts were calculated from autocorrelation functions for the hydration shells of the ions.

  • 35. Lu, Zhansheng
    et al.
    Kullgren, Jolla
    Yang, Zongxian
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Sulfidation of Ceria Surfaces from Sulfur and Sulfur Diffusion2012Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, nr 15, s. 8417-8425Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Even very low levels of sulfur contaminants can degrade the catalytic performance of cerium oxide. Here, the interaction of atomic sulfur with the ceria (111) and (110) surfaces has been studied using first-principles methods. Two sulfoxy species are identified: oxido-sulfate(2-) species (SO2-) on both the CeO2(111) and (110) surfaces and hyposuffite (SO22-) on the (110) surface. Sulfide (S2-) is formed when a surface or a subsurface oxygen atoms is replaced by sulfur. These sulfide species are more stable at the surface. Furthermore, sulfite (SO32-) structures are found when sulfur is made to replace one Ce in the ceria (111) and (110) surfaces. The calculated sulfur diffusion barriers are larger than 1.4 eV for both surfaces, and thus sulfur is essentially immobile, providing a possible explanation for the sulfidation phenomena of the ceria-based catalyst. Thus, we find three different species from interaction of S with ceria which are all, due to their strong binding, capable of poisoning the surface, reduced or unreduced. Our results suggest that under reducing conditions sulfur is likely to be found in the (111) surface (replacing oxygen) but on the (110) surface (as SO22-).

  • 36. Lu, Zhansheng
    et al.
    Yang, Zongxian
    He, Bingling
    Castleton, Christopher
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Cu-doped ceria: Oxygen vacancy formation made easy2011Inngår i: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 510, nr 1-3, s. 60-66Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    DFT + U calculations of Cu-doped bulk ceria are presented. The first oxygen vacancy in Cu-doped ceria forms almost spontaneously and the second vacancy is also easily created. Whether zero, one or two oxygen vacancies, the Cu dopant is in the form Cu(+ II), and prefers to be 4-coordinated in a close to planar structure. Charge compensation, structural relaxation and available Cu-O states all play roles in lowering the O vacancy formation energies, but to different degrees when the first and second oxygen vacancies are formed. The Cu-doped ceria(1 1 1) surface system behaves in a similar fashion.

  • 37. Lu, Zhansheng
    et al.
    Yang, Zongxian
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Castleton, Christopher W. M.
    Several different charge transfer and Ce3+ localization scenarios for Rh-CeO2(111)2014Inngår i: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 2, nr 7, s. 2333-2345Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present DFT+U based electronic structure calculations in a p(3 x 3) slab supercell, for low coverages of atomically dispersed Rh interacting with the CeO2(111) surface, comparing Rh as an adatom, and as a dopant substituted into the surface layer. We find that, energetically, a Rh atom approaching a ceria(111) surface with both sparse O and Ce vacancies present strongly prefers to heal the Ce vacancies, but next it prefers to adsorb on a stoichiometric region rather than healing an O vacancy. In the adatom system, Rh is oxidized by electron transfer to a 4f orbital on one Ce ion in the surface layer, which is then nominally converted from Ce4+ -> Ce3+ (i.e. Rh adatoms are single donors). We show that there are a number of different local minima, with Ce3+ localization at 1st, 2nd or 3rd nearest neighbour Ce sites. The second neighbour is the most stable, but all are close in energy. In the Rh-doped system (Rh replaces Ce), Rh is oxidized by charge transfer to neighbouring O atoms, and Rh doping leads to deep acceptor and donor states. Rh is not stable in the O sublattice. Moreover, based on vacancy formation energies, we find that oxygen vacancy formation is strongly enhanced in the vicinity of Rh dopants, but slightly suppressed in the vicinity of Rh adatoms.

  • 38.
    Mitev, Pavlin D.
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Bopp, Philippe A.
    Petreska, Jasmina
    Coutinho, Kaline
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Pejov, Ljupco
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Different structures give similar vibrational spectra: The case of OH- in aqueous solution2013Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, nr 6, s. 064503-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have calculated the anharmonic OH-(aq) vibrational spectrum in aqueous solution with a "classical Monte Carlo simulation + QM/MM + vibrational" sequential approach. A new interaction model was used in the Monte Carlo simulations: a modified version of the charged-ring hydroxide-water model from the literature. This spectrum is compared with experiment and with a spectrum based on CPMD-generated structures, and the hydration structures and H-bonding for the two models are compared. We find that: (i) the solvent-induced frequency shift as well as the absolute OH- frequency are in good agreement with experiment using the two models; (ii) the Raman and IR bands are very similar, in agreement with experiment; (iii) the hydration structure and H-bonding around the ion are very different with the two ion-water interaction models (charged-ring and CPMD); (iv) a cancellation effect between different regions of the hydration shell makes the total spectra similar for the two interaction models, although their hydration structures are different; (v) the net OH- frequency shift is a blueshift of about + 80 cm(-1) with respect to frequency of the gas-phase ion.

  • 39. Mitev, Pavlin D.
    et al.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Surface properties of rutile TiO2(110) from molecular dynamics and lattice dynamics at 300 K: Variable-charge model results2007Inngår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 601, nr 23, s. 5359-5367Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Geometric structure, atomic vibrations and atomic charges and their thermally induced fluctuations have been calculated as a function of depth in, and thickness of, rutile TiO2(110) slabs, within the framework of the variable-charge potential of Swamy and Gale [V. Swamy, J.D. Gale, Phys. Rev. B 62 (2000) 5406] at 300 K. Molecular dynamics simulations and lattice dynamics calculations were performed with a 2D periodic slab model for slab thicknesses between 3 and 11 triple layers (approximately 9-35 angstrom). Odd-even oscillations with respect to the number of slab layers are found for the surface relaxation for very thin slabs, and for the (slowly converging) rumpling in the middle of the slab. The Ti and O atomic charges in the outermost three atomic layers differ from the rest of the slab (they are less ionic); the thermal vibrations do not alter this picture. The atomic mean-square amplitudes are some 50% larger (more for O, less for Ti) at the surface than in the middle of the slab and decay rather slowly to the bulk values. Comparisons with the results of a rigid-ion potential for titania [M. Matsui, M. Akaogi, Mol. Simul. 6 (1991) 238] are presented for non-electronic properties.

  • 40.
    Mitev, Pavlin
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Briels, Wim J.
    Calculation of anharmonic OH phonon dispersion curves for the Mg(OH)(2) crystal2010Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 133, nr 3, s. 034120-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Anharmonic OH phonon dispersion curves have been calculated for the Mg(OH)(2) crystal. A crystal Hamiltonian was set up for the vibrational problem, where the coordinates consists of the bond lengths of two hydroxide ions in the central unit cell. Its two-dimensional potential energy surface was constructed from first principle calculations within the density functional theory approximation. Dispersion curves were calculated by diagonalizing the Hamiltonian in a basis of singly excited crystal functions. The single particle functions used to construct the crystal states were taken from a Morse oscillator basis set. These well chosen functions made it possible to restrict calculations to include only very few functions, which greatly contributed to a transparent presentation of the underlying theory. All calculations could be done analytically except for the calculation of a few integrals. We have compared our results with those of a series of harmonic lattice dynamics calculations and have found that the anharmonicity shifts the IR and Raman dispersion curves downward appreciably and slightly changes the energy differences between both curves. From an analysis of the harmonic results we conclude that incorporating the coupling between OH stretching motion and the motion of their centers of mass will appreciably change the overall features of the dispersion curves. Extension of the anharmonic model along these lines will cause no problem to the theoretical approach presented in this paper.

  • 41.
    Mitev, Pavlin
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Montanari, Barbara
    Refson, Keith
    Soft modes in strained and unstrained rutile TiO22010Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, nr 13, s. 134303-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have studied the lattice dynamics of crystalline TiO2 using density-functional perturbation theory and the local-density approximation in a plane-wave pseudopotential formalism at equilibrium and uniaxially strained geometries. We present well-converged calculations of the dispersion curves, which sample a more complete volume of the Brillouin zone than in previous studies. We find an anomalously soft TA mode in a region of reciprocal-space previously unexplored either by any previous calculation or experiment. This is quite separate from the A2u mode which becomes soft at the point and is responsible for the incipient ferroelectric behavior. The harmonic frequency of the soft TA mode around q = (1/2, 1/2, 1/4) decreases to zero under an isotropic expansion with a strain slightly above 0.5% and we suggest that it may be possible to observe anomalous diffuse inelastic scattering corresponding to a dynamical instability using neutron scattering. In addition to the softening under isotropic strain, the frequency of this mode goes to zero under uniaxial strain along the [110] direction in both compression and expansion (at close to -0.5% and + 1.0%, respectively which offers new possibilities for experimental tests of softening under compressional strain. We further suggest that the soft TA mode may help explain the anomalously long-ranged convergence observed in previous calculations on slab models of the TiO2 (110) surface by providing a mechanism for small changes in bonding at the surface to propagate deep into the bulk. The behavior of other modes under strain was also studied. The ferroelectric A(2u) mode frequency is nearly independent of [110] strain, which contrasts with the behavior in response to [001] strain reported in the literature of a strong dependence. However, the frequency of the Raman-active B-2g mode does decrease to zero frequency under 1.3% strain, which should be observable using Raman spectroscopy.

  • 42. Muller, C.
    et al.
    Freysoldt, C.
    Baudin, M.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    An ab initio study of CO adsorption on ceria(110)2005Inngår i: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 318, nr 3, s. 180-190Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Hartree-Fock and DFT calculations are reported for the CO/CeO2(110) surface system. The electron density, electrostatic potential, atomic charges and projected electronic density of states have been calculated from an ECP-and-point-charge-embedded cluster model and is compared with periodic calculations. The agreement between the two surface models is reasonably good. A number of weakly bonding CO adsorption sites were found, with E-ads (BSSE-corrected) ranging from 0.01 to 0.22 eV per adsorbed molecule. The two most favourable sites are found in the vicinity of surface cerium ions, with the CO molecule oriented in a tilted fashion, C-end down. The surface-induced CO stretching vibrational frequency shifts on these sites are a redshift of approximate to-30 cm(-1) and a blueshift of approximate to 25 cm(-1), respectively.

  • 43. Nokbin, S.
    et al.
    Limtrakul, J.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    DFT plane-wave calculations of the Rh/MgO(001) interface2004Inngår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 566, s. 977-982Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Rh/MgO(001) system has been studied by periodic plane-wave density functional calculations using the VASP code and PAW potentials. Four different adsorption sites (which were reduced to three after optimization) and three different surface coverages were investigated. For the most stable site, above O, the adhesion energy was found to decrease as a function of coverage (from 2.0 to 1.1 eV as the coverage increases from 1/8 to I ML), while the adsorption energy was found to increase with surface coverage. Electron density difference plots were calculated to display some of the electron rearrangement responsible for the Rh-oxide adhesion energy, and the features of the Mg and O adsorption sites were compared.

  • 44. Onthong, U.
    et al.
    Bako, I.
    Radnai, T.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Probst, M.
    Ab initio study of the interaction of dimethylsulfoxide with the ions Li+ and I2003Inngår i: International Journal of Mass Spectrometry, ISSN 1387-3806, E-ISSN 1873-2798, Vol. 223, nr 03-jan, s. 263-270Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report analytical expressions for the dimethylsulfoxide (DMSO)-Li+ and DMSO-I- potential energy hypersurfaces. Quantum chemically calculated energies were fitted to polynomial functions of different number of parameters. These calculations are augmented by calculations of the electrostatic potential in the vicinity of DMSO. Global geometry optimizations and calculations of ion-induced frequency shifts have been performed. We find that at its global energy minima, Li+ in the DMSO-Li+ complex is bound to the oxygen atom with an energy of about 2.6 eV while I- in the vicinity is bound between the two methyl groups in a shallow energy minimum with about 0.6 eV.

  • 45. Onthong, U.
    et al.
    Megyes, T.
    Bako, I.
    Radnai, T.
    Grosz, T.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Probst, M.
    X-ray and neutron diffraction studies and molecular dynamics simulations of liquid DMSO2004Inngår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 6, nr 9, s. 2136-2144Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Liquid DMSO has been investigated by means of X-ray and neutron diffraction, quantum-chemical calculations and molecular dynamics (MD) simulations. A new analytical all-atom pair potential for DMSO-DMSO interactions was developed from quantum-chemical calculations and employed in the simulations. The MD-derived total structure function agrees favourably with those obtained from the X-ray and neutron diffraction experiments. An analysis of the local structure shows a rather unstructured liquid with a slight preference for antiparallel ordering of the S-O dipoles.

  • 46. Onthong, U.
    et al.
    Megyes, T.
    Bako, I.
    Radnai, T.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Probst, M.
    Molecular dynamics simulation of lithium iodide in liquid dimethylsulfoxide2005Inngår i: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 401, nr 03-jan, s. 217-222Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report results of a molecular dynamics simulation on a system consisting of 343 DMSO molecules, one Li+ cation and one I- anion in the NVE ensemble at 310 K, employing our recently developed all-atom potential functions for the intermolecular interactions derived from quantum chemical calculations. The structure of the solvation shell around the cation and the anion are investigated in terms of radial and angular distribution functions. The first solvation shell around lithium ion consists of 4 DMSO molecules; 8 DMSO molecules coordinate to iodide ion. DMSO coordinates to both ions in a well-defined way. The results are in good agreement with other molecular dynamics simulations of alkali halides in DMSO.

  • 47. Pejov, L.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    MD plus QM calculations of the OH vibrational stretching band in an aqueous aluminium(III) chloride solution2002Inngår i: Journal of Molecular Liquids, ISSN 0167-7322, E-ISSN 1873-3166, Vol. sep-98, s. 367-380Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The OH stretching vibrational band has been calculated by an 'MD+QM' approach for the water molecules in the first hydration shell of the Al3+ ion in a dilute AlCl3 aqueous solution. Molecular Dynamics-generated geometrical snapshots provided the molecular positions, which were subsequently used in ab initio calculations of charge-embedded ion-water clusters at the HF and B3LYP levels. The ion-water clusters which were treated quantum-mechanically were Al3+(H2O)(6)(H2O)(x) (x=2 or 3), where a few water molecules from the second hydration shell were included. 60 oscillators were selected from the MD dumps, and anharmonic vibrational frequencies were computed from the ab initio-generated potential energy curves. Density-of-states bands as well as infrared-intensity-weighted bands are presented. Our average frequency downshift is only 60% of the experimental downshift. Further refinement of the method will concentrate on the description of the remaining part of the second, and the third, hydration shells.

  • 48. Pejov, L.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    On the nature of blueshifting hydrogen bonds: Ab initio and density functional studies of several fluoroform complexes2003Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 119, nr 1, s. 313-324Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Potential energy hypersurfaces (PESs) for four fluoroform complexes (with acetonitrile, ethyleneoxide, formaldehyde, and water) were explored at the HF, MP2, and B3LYP/6-311 ++G(d,p) levels of theory. Anharmonic C-H stretching vibrational frequency shifts are reported for all minima located on the studied PESs. In all cases, the lowest-energy minimum occurs for a C-H...O(N) hydrogen-bonded arrangement and is characterized by a significant C-H frequency blueshift (upshift), while additional minima [for reversed'' orientations, in which there is no direct C-H...O(N) contact] show only small C-H frequency upshifts. The large blueshifts found for the hydrogen-bonded arrangements are predominantly caused by the electronic exchange interaction, as revealed by Kitaura-Morokuma (KM) analysis, while the purely electrostatic + polarization interaction leads to C-H frequency redshifts, which was proven both by the KM analysis and the charge field perturbational (CFP) approach. The large net blueshifting effect of the exchange contribution is only possible thanks to the smallness of the redshifting electrostatic + polarization contribution, which, in turn, is a consequence of the fact that dmu((0))/dr(CH) is negative for the fluoroform molecule. In all cases the charge transfer as well as the dispersion energy contributions lead to frequency redshifts. The small C-H blueshifts for the reversed orientations are almost completely governed by the electrostatic interaction, as shown by the KM and CFP analyses. All these characteristics of blueshifting H bonds are in line with a previously outlined general model concerning standard'' and blueshifting'' H-bonding interactions

  • 49. Pejov, L.
    et al.
    Spangberg, D.
    Hermansson, Kersti
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Using MD snapshots in ab initio and DFT calculations: OH vibrations in the first hydration shell around Li+(aq)2005Inngår i: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 109, nr 23, s. 5144-5152Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The average OH stretching vibrational frequency for the water molecules in the first hydration shell around a Li+ ion in a dilute aqueous solution was calculated by a hybrid molecular dynamics + quantum-mechanical (MD + QM) approach. Using geometry configurations from a series of snapshots from an MD simulation, the anharmonic, uncoupled OH stretching frequencies were calculated for 100 first-shell OH oscillators at the B3LYP and HF/6-31G(d,p) levels of theory, explicitly including the first shell and the relevant second shell water molecules into charge-embedded supermolecular QM calculations. Infrared intensity-weighting of the density-of-states (DOS) distributions by means of the squared dipole moment derivatives (which vary by a factor of 20 over the OH stretching frequency band at the B3LYP level), changes the downshift from approximately -205 to -275 cm(-1) at the B3LYP level. Explicit inclusion of relevant third-shell water molecules in the supermolecular cluster leads to a further downshift by approximately -30 cm-1. Our final estimated average downshift is approximately -305 cm-1. The experimental value lies somewhere in the range between - 290 and -420 cm-1. Also, the absolute nu(OH) frequency is well reproduced in our calculations. In-liquid instantaneous correlation curves between nu(OH) and various typical H-bond strength parameters such as R((OO)-O-...), R((HO)-O-...), the intramolecular OH bond length, and the IR intensity are presented. Some of these correlations are robust and persist also for the rather distorted instantaneous geometries in the liquid; others are less so.

  • 50. Probst, M.
    et al.
    Hermansson, Kersti
    KTH, Tidigare Institutioner                               , Bioteknologi.
    Urban, J.
    Mach, P.
    Muigg, D.
    Denifl, G.
    Fiegele, T.
    Mason, N. J.
    Stamatovic, A.
    Mark, T. D.
    Ionization energy studies for ozone and OClO monomers and dimers2002Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 116, nr 3, s. 984-992Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Electron impact ionization cross sections measured close to threshold are reported for both the monomers and dimers of ozone and OClO using a new high resolution electron impact apparatus. The present appearance energies AE(O-3(+)/O-3)=12.70 +/-0.02 eV, AE (OClO+/OClO)=10.55 +/-0.02 and AE(ClO+/OClO=13.37 +/-0.03 eV derived from the measured ionization cross sections are in excellent agreement with the vertical threshold values determined for these ions by high resolution PES and PIMS photoionization studies. The corresponding appearance energies determined for the dimer ions, 10.10 +/-0.3 eV for (O-3)(2)(+) and 9.87 +/-0.2 eV for (OClO)(2)(+), are both red shifted with respect to the monomer case. The bond energy (0.70-0.3+0.5) eV of (OClO)(2)(+) estimated from these data is similar to that of other dimer ions, whereas the bond energy of (O-3-O-3(+)) with (2.55-0.4+0.6) eV is rather large suggesting an unusual structure for the cationic ozone dimer ion. Based on quantum chemical calculations on various levels we are led to the conclusion that the ion produced by ionization of the ozone dimer is no longer a conventional dimer ion where the two monomer units are still present (as is the case for the OClO system), but rather an ion of form O-2.O-4(+) or a twisted ring structure of (O-6)(+).

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