Simulation and theory of fluids of axisymmetric molecules

Abstract

Molecular dynamics simulations have been performed to obtain pressure, internal energy, and residual Helmholtz free energy for gaussian overlap fluids. Simulation results have been used to test an existing perturbation theory and a proposed sphericalized potential method for these fluids. Serious deviations between perturbation theory and simulation are observed. Simulations using the reference potential show that while the first order Helmholtz free energy expansion converges for these fluids, approximations made in perturbation theory to obtain the pair distribution function are not very good at high densities and lead to underprediction of the first order term in the Helmholtz free energy expansion. The sphericalized potential method, however, shows improvements over the perturbation theory for the Helmholtz free energy and the pressure of fluids of prolate molecules and for the pressure of fluids of oblate molecules. Comparisons have also been made for the case of Lennard-Jones quadrupolar fluids and the sphericalized potential method is shown to predict well the residual Helmholtz free energy of simple quadrupolar fluids in spite of its simplicity.