INTEGRAL-EQUATION THEORY OF MOLECULAR LIQUIDS - KIRKWOOD HIERARCHY APPROACH TO DIATOMIC AND POLYATOMIC LIQUIDS

A set of integral equations is derived for site-site pair correlation functions for diatomic fluids from the Kirkwood hierarchy of correlati on functions. The integral equations obtained are self-consistent fiel d equations which are generalizations of the Percus-Yevick integral eq uation for simple liquids. In this theory the intra- and intermolecula r correlation functions are self-consistently coupled. The integral eq uations reduce to the integral equations in the reference site interac tion model (RISM) theory of diatomic liquids if an approximation is ma de to the direct correlation functions and the molecule is assumed to be rigid. The theory is also generalized to polyatomic fluids. The pre dictions of the theory are compared with those by the RISM theory and Monte Carlo simulations available in the literature. The computational requirement for the integral equations of the present theory is compa rable to that of the RISM theory, but its predictions are more accurat e than those by RISM theory at low densities and are practically the s ame as the latter at high densities. (C) 1996 American institute of Ph ysics.