Algebraic perturbation theory for dense liquids with discrete potentials

A simple theory for the leading-order correction g_1(r) to the structure of a hard-sphere liquid with discrete (e.g. square-well) potential perturbations is proposed. The theory makes use of a general approximation that effectively eliminates four-particle correlations from g_1(r) with good accuracy at high densities. For the particular case of discrete perturbations, the remaining three-particle correlations can be modeled with a simple volume-exclusion argument, resulting in an algebraic and surprisingly accurate expression for g_1(r). The structure of a discrete "core-softened" model for liquids with anomalous thermodynamic properties is reproduced as an application.