Molecular dynamics study of pressure in molecular systems

Abstract

In molecular dynamics simulation of molecular systems, an atomistic model is needed to describe the intramolecular effects on system properties such as pressure. An expression for computing the pressure is derived based on the virial theorem, with explicit kinetic, intra-, and intermolecular contributions. It is shown that the virial terms arising from three- or four-body forces that depend only on internal angles are zero by using the Wilson S vector technique, and that only the two-body forces appear in the pressure expression. Molecular dynamics simulations are carried out for an atomistic model of benzene with intramolecular interactions based on ab initio harmonic potentials and intermolecular interactions given by semiempirical atom–atom potentials. Calculated total pressure depends on parameters of intermolecular potentials. An intramolecular contribution to the pressure is studied for both solid and liquid phases. In solids where the molecules are compressed, the intramolecular pressure contributions are appreciable and positive, while in liquids where molecular deformations are relatively small, the contributions are small and negative. A possible further improvement of the atomistic model of benzene is discussed.