DENSITY-FUNCTIONAL PERTURBATION-THEORY OF INHOMOGENEOUS SIMPLE FLUIDS

The mean-field approximation, widely used in the nonlocal-density-functional theories of inhomogeneous simple fluids, is reexamined. Comparing its predictions of the density profiles of a Lennard-Jones fluid near a hard wall with those of Monte Carlo simulations reveals that the approximation is qualitatively incorrect at low densities and quantitatively inaccurate at intermediate and high densities. A density-functional perturbation theory is proposed. It combines the nonlocal-density-functional model of an inhomogeneous hard-sphere system with the Barker-Henderson second-order perturbation theory of uniform simple fluids. It also takes into account the softness of the short-range repulsive potential. The new theory is shown to be qualitatively correct and quantitatively more accurate over the whole range of liquid densities. The effects of the pair potential truncation and the self-consistency of the nonlocal-density-functional theories are discussed.