An interpretation of the mean spherical approximation

The mean spherical approximation is shown to be derivable from a perturbation expansion where the reference system follows the Percur-Yevick approximation. Then the MSA may be interpreted as a first-order expansion in which the perturbation is averaged with respect to the low-density reference distribution function. This represents a generalization of an equivalent interpretation previously made with respect to the free energy of a polar fluid.