A liquid-state theory that remains successful in the critical region

Abstract

A thermodynamically self-consistent Ornstein—Zernike approximaton (SCOZA) is applied to a fluid of spherical particles with a pair potential given by a hard core repulsion and a Yukawa attractive tail w(r) =—exp[—z(r—1)]/r. This potential allows one to take advantage of the known analytical properties of the solution of the Ornstein—Zernike equation for the case in which the direct correlation function outside the repulsive core is given by a linear combination of two Yukawa tails and the radial distribution function g(r) satisfies the exact core condition g(r) = 0 for r < 1. The predictions for the thermodynamics, the critical point, and the coexistence curve are compared with other theories and with simulation results. In order to assess unambiguously the ability of the SCOZA to locate the critical point and the phase boundary of the system, a new set of simulations also has been performed. The method adopted combines Monte Carlo and finite-size scaling techniques, and is especially adapted to deal with critical fluctuations and phase separation. It is found that the version of the SCOZA considered here provides very good overall thermodynamics and remarkably accurate critical point and coexistence curve. For the interaction range considered here, given by z = 1.8, the critical density and temperature predicted by the theory agree with the simulation results to about 0.6%.