Structures of liquids with non-central interactions

Abstract

At present, two different methods are employed to take into account the structural contribution of noncentral intermolecular interactions. The first is the perturbation method, which should work well at not too high densities; the second method tries to separate in an approximate way the configurational part and the orientational part of the partition function. The perturbation method can be varied by making different choices of the reference system and the perturbing potential, and some significant choices will be outlined. If the non—central part of the intermolecular interaction is a dipolar part only, the density dependence of the Kirkwood g-factor should be positive in first order perturbation. Structures like in fluid acetonitrile, where the densitydependence of the Kirkwood g-factor changes sign, can only be explained by a more complicated intermolecular interaction, or by the unknown higher order terms in the perturbation scheme. The approximate factorization of the partition function into a configurational and an orientational part is less developed, but looks very promising, especially for getting insight into the behaviour of a liquid at high densities, up to triple point densities. INTRODUCTION Before treating liquids composed of molecules with non-central forces, let me briefly recapitulate the theory of central-force liquids. The main breakthrough of the last 15 years was the recognition that the structure of fluids at high density is solely determined by the repulsive branch of the intermolecular interaction (1-4). This is particularly true if long-range forces are absent, and if the division of the potential into a repulsive and an attractive branch is performed according to the suggestion of Weeks, Chandler and Andersen (3) (fig.1). One might visualize that the attractive branches of the potentials, which the neighbouring molecules exert on the central molecule, superpose to a homogeneous background potential (fig.2).