Perturbation theory for diatomic molecules using a non-spherical reference system

Abstract

A perturbation theory for predicting the thermodynamic properties and structure of a molecular liquid is described. The theory is applied to a two-centre Lennard-Jones fluid and the properties of this fluid are calculated from the properties of a hard dumbell fluid. The liquid structure of the dumbell reference system is calculated using the RISM equations. The free energy of the reference system is calculated by scaled particle theory and by Monte Carlo simulation. The non-spherical hard-core system is softened before the anisotropic perturbation is added. The theory makes qualitative predictions about the structure of molecular liquids and quantitative predictions of the internal energy, pressure and mean squared force. The perturbation expansion is rapidly convergent and the principal source of error is the reference system distribution function. The extension to more complex models and the addition of multipole interactions is discussed.