Use of background correlations in statistical thermodynamic calculations for non-spherical molecules

Abstract

In calculations of thermodynamic quantities such as the energy and pressure of an ensemble of non-spherical molecules, it has been suggested that the relevant molecular properties such as intermolecular pair potential and correlation function can be conveniently represented by writing their dependence upon orientation as expansions in spherical harmonics. However, recent computer simulations have shown that the resulting series for the thermodynamic properties do not converge rapidly for physically realistic non-spherical molecular models. An alternative expansion is suggested which is based on the background part of the correlation function rather than the correlation function itself. Thermodynamic properties of non-spherical molecules can be calculated from the spherical harmonic expansion coefficients of the background correlation; however, the parts of the integrands involving the pair potential energy and its derivatives must be appropriately altered. Explicit calculations are presented for a molecular model and it is argued that the new expansions should converge more rapidly than those used in previous work.