An extension of the Weeks—Chandler—Andersen perturbation theory, proposed by Wright and Perram is applied to liquid Na By neglecting the potential beyond its first minimum, the structure factor is found to be given accurately except in the vicinity of the first peak.

A parametric representation of the radial distribution function of a simple liquid is proposed. The Percus-Yevick approximation is solved for this system yielding an interaction potential which is then used as a reference system within the Weeks, Chandler and Andersen “blip function” theory.