Cluster perturbation theory for interaction site fluids

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Abstract
We describe a method for determining the contributions to the structure and thermodynamics of interaction site fluids arising from long-ranged perturbations to the site–site potential. An extension of cluster perturbation theories developed for atomic fluids (optimized cluster theory, Γ ordering) to the interaction site cluster expansion is the basis of the new theory. Given the pair distribution function and the Helmholtz free energy of the reference system, the theory predicts the contribution to the structure and Helmholtz free energy arising from a perturbation to the potential, and contains no nonphysical contributions such as those arising in previous theories for these systems. Various levels of approximation are possible within the theory. We discuss the lowest level in detail. This is the extension of the optimized random phase approximation to the interaction site formalism (ISF-ORPA). The relationship with integral equation theories for these systems is established.