Density Expansions for Effective Pair Potentials

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Abstract
The leading density dependence for the variationally defined effective pair potential v is derived, using van Kampen's product technique to evaluate the multidimensional configuration integrals that arise. This function v is compared in that density order to two other effective pair potentials that have been proposed, and it is shown to be the only one that simultaneously reduces both free energy and pair correlation errors to quadratic order in the magnitude of triplet nonadditivity. The signs of density shifts in v at low temperature are analyzed for noble gases and for water in various configurations, and a reasonable arbitrary‐density approximation to v is suggested that may have utility in simulation of liquids by rapid electronic computer.