A model for the computer simulation of chemical reactions in the condensed phase

Abstract

Molecular dynamics calculations have been performed on small numbers of atoms (4-64), contained within a deformable boundary, with a view to developing a model suitable for simulating chemical reactions in the condensed phase. To determine how realistic the model is, a series of standard thermodynamic quantities have been evaluated. Velocity distributions have been determined during the course of individual simulations as a function of temperature and number of atoms. For 16 atoms or more, these distributions compare very favourably with a Maxwellian distribution. An effective pressure has been calculated by two methods, one using a virial of the internal forces and another in which the forces exerted by an external container have been calculated. In the case of the latter there is a lengthy induction period before the pressure stabilizes. For 64 argon atoms at 100 K, a calculated PV curve shows evidence of condensation. For the same number of atoms at 200 K, the PV curve fits a van der Waals expression whose constants are close to experimental values. The model has been tested on the formation and decomposition of Xe₂ in an argon heat bath, and the calculated equilibrium constant for Xe dimerization compares favourably with that deduced from statistical mechanics.