Perturbation correction for the free energy and structure of simple fluid mixtures

Adapting an analysis due to Morita and Hiroike and Green which led to the hypernetted-chain (HNC) integral equation, we obtain an expression for the difference in free energy between a perturbed mixture with potentials φα β = φα β(0) + φα β(1) and a reference mixture with potentials φα β(0) a result which is rendered computable by an HNC-type approximation. No further approximations are required to compute the corrected pair distribution functions of the perturbed mixture, but a proposed second approximation yields a particularly simple solution. This work generalizes earlier results on single-component fluids.