The statistical mechanics of assemblies of axially symmetric molecules - I. General theory

Abstract

A general method for investigating the thermodynamic effects of the dependence of the intermolecular potential on orientation is described. The intermolecular potential for axially symmetric molecules is expanded in such a way that the central-force part can be separated uniquely and the remaining directional parts classified in terms of various angular symmetries. Several simple types of directional intermolecular force are treated in this way as examples. The free energy of an assembly of such systems is then calculated by treating the directional parts of the field as a perturbation on the central-force part. This leads to a simple expression for the extra free energy due to directional forces in terms of the molecular distribution functions of an assembly of molecules interacting according to a central-force law. A corresponding treatment of multicomponent systems is also given.