The capacitive argon discharge operated in the intermediate pressure regime is studied by performing one-dimensional particle-in-cell Monte Carlo collision simulations. First, the basic object-oriented plasma device code (oopd1-v1) is strictly benchmarked against the well-established xpdp1 code over a wide range of pressure (0.05-15 Torr) and varying blocking capacitor of the external circuit (5-10(5) nF), and excellent agreement is obtained. The oopd1-v1 is upgraded to oopd1-v2 and oopd1-v3, by adding excited atoms modeled as time- and space-evolving fluid species. The metastable Ar-m, the radiative Ar(R), and the Ar(4p) manifold, and their roles in discharge equilibrium are explored. It is found that the presence of the metastable Ar-m enhances the plasma density by a factor of 3 at 1.6 Torr and even higher at pressures up to 5 Torr. At low pressure (0.05 Torr), electron impact ionization from the ground state atom dominates the ionization over the whole discharge region, while metastable pooling and step-wise ionization has small contribution. The proportion of metastable pooling ionization and step-wise ionization increases with increasing pressure and becomes the dominant ionization source at 5-15 Torr.