A complete integral equation formulation in the interaction site formalism

Abstract

An integral equation for the full particle-particle correlation functions in molecular fluids is obtained by expressing the relevant quantities in terms of the graphical language of the interaction site formalism. The equation is of the form of the Ornstein-Zernike equation for an atomic mixture, but is mathematically equivalent to the ‘proper’ integral equation of Chandler, Silbey, and Ladanyi. The simple form results in part from the introduction of a convenient method which yields the topologically correct interaction site graph chain sums. Further, by direct analogy with the usual graphical analysis of atomic fluids, we obtain a closure relation which is formally exact in the same sense as that in the usual atomic theory. Numerical results obtained using closures which are the graphical analogues of the usual atomic Percus-Yevick and hypernetted chain approximations are discussed for model homonuclear Lennard-Jones diatomic fluids. These preliminary results suggest that such direct generalizations are not likely to provide improvement over RISM results.