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  • 1.
    Lundberg, Linnea
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
    Dispersion Corrections at Planar Surfaces2016Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    When simulating a molecular system, a cutoff distance for interactions is often used to speed up the simulations. This is made at the cost of neglecting some interactions which will lead to inaccurate results for energy, pressure components and surface tension (for systems with surfaces). To compensate for the neglected long-range interactions, continuum corrections can be added to the surface tension, system energies and pressures. For a homogenous isotropic system this is straight-forward but for a system with a surface it is more complicated. In this work we have derived expressions for the corrections to the surface tension, system energies and pressures that are more general than previous results. When these corrections are added to multi-component systems with a surface (or single-component systems with vacuum) they compensate for the change in surface tension, system energy and pressures due to the finite cutoff. When simulating systems with no Coulomb-interactions, the structure of the system may change significantly if the cutoffs are too short. If this is the case then these corrections alone will not be enough. The solution is to add corrections to the force acting on each molecule added during the simulation, which we derive in this work. This solves the structural problem at low cutoffs and makes it possible to calculate an accurate surface tension independent of cutoff. 

  • 2.
    Lundberg, Linnea
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
    Edholm, Olle
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
    Dispersion Corrections to Forces at Planar SurfacesManuscript (preprint) (Other academic)
    Abstract [en]

    It is known that a cutoff on the van der Waals interactions in molecular dy- namics simulations of systems containing a surface (gas/liquid for instance) may cause substantial changes in the densities of the different parts of the system. In order to make the density independent of the cutoff, we have calculated forces from continuum integrations and added them to the forces calculated in a molecular dynamics program. These corrections compensate for the lack of long range interactions and exert a pressure on the system that may be several hundred bars. We show here that inclusion of such corrections makes the density profile independent of the cutoff. This makes it possible to calculate a surface tension that is independent of the cutoff. 

  • 3.
    Lundberg, Linnea
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
    Edholm, Olle
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
    Dispersion Corrections to the Surface Tension at Planar Surfaces2016In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 12, no 8, p. 4025-4032Article in journal (Refereed)
    Abstract [en]

    Molecular dynamics simulations are usually performed using cutoffs (r(c)) for the short-ranged dispersion interactions (r(-6)). For isotropic systems, long-range interactions are often added in a continuum approximation. This usually leads to excellent results that are independent of the cutoff length down to about 1 nm. For systems with interfaces or other anisotropic systems the situation is more complicated. We study here planar interfaces, focusing on the surface tension, which is sensitive to cutoffs. Previous analytic results giving the long-range correction to the surface tension of a liquid-vapor interface as a two- or three-dimensional integral are revisited. They are generalized by introducing a dispersion density profile which makes it possible to handle multicomponent systems. For the simple but common hyperbolic tangent profile the integral may be Taylor-expanded in the dimensionless parameter obtained by dividing the profile width with the cutoff length. This parameter is usually small, and excellent agreement with numerical calculations of the integral is obtained by keeping two terms in the expansion. The results are compared to simulations with different lengths of the cutoff for some simple systems. The surface tension in the simulations varies linearly in r(c)(-2), although a small r(c)(-4)-term may be added to improve the agreement. The slope of the r(c)(-2)-line could in several cases be predicted from the change in dispersion density at the interface. The disagreements observed in some cases when comparing to theory occur when the finite cutoff used in the simulations causes structural differences compared to long-range cutoffs or Ewald summation for the r(-6)-interactions.

  • 4.
    Lundberg, Linnea
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
    Edholm, Olle
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
    Dispersion Corrections to the Surface Tension at Planar SurfacesManuscript (preprint) (Other academic)
    Abstract [en]

    Molecular dynamics simulations are usually performed using cutoffs (rc) for the short-ranged dispersion interactions (r−6). For isotropic systems, long-range interactions are often added in a continuum approximation. This usually leads to excellent results that are independent of the cutoff length down to about 1 nm. For systems with interfaces or other anisotropic systems the situation is more complicated. We study here planar interfaces, focusing on the surface tension, which is sensitive to cutoffs. Previous analytic results giving the long-range correction to the surface tension of a liquid-vapor interface as a two- or three-dimensional integral are revisited. They are generalized by introducing a dispersion density profile which makes it possible to handle multi-component systems. For the simple but common hyperbolic tangent profile the integral may be Taylor-expanded in the dimensionless parameter obtained by dividing the profile width with the cutoff length. This parameter is usually small and excellent agreement with numerical calculations of the integral is obtained by keeping two terms in the expansion. The results are compared to simulations with different lengths of the cutoff for some simple systems. The surface tension in the simulations varies linearly in rc−2, although a small rc−4-term may be added to improve the agreement. The slope of the rc−2-line could in several cases be predicted from the change in dispersion density at the interface. The disagreements observed in some cases when comparing to theory, occur when the finite cutoff used in the simulations causes structural differences compared to long-range cutoffs or Ewald summation for the r−6-interactions. 

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