ROLES OF REPULSIVE AND ATTRACTIVE FORCES IN DETERMINING THE STRUCTURE OF NONUNIFORM LIQUIDS : GENERALIZED MEAN FIELD THEORY
The structure of a nonuniform Lennard-Jones (LJ) liquid near a hard wall is approximated by that of a reference fluid with repulsive intermolecular forces in a self-consistently determined external mean field incorporating the effects of attractive forces. We calculate the reference fluid structure by first determining the response to the slowly varying part of the field alone, followed by the response to the harshly repulsive part. Both steps can be carried out very accurately, as confirmed by Monte Carlo simulations, and good agreement with the structure of the full LJ fluid is found. [S0031-9007(98)07606-6] PACS numbers: 61.20.Gy, 68.10.Cr, 68.45.Gd Dense liquids have highly nontrivial density correlations arising because the harshly repulsive molecular cores cannot overlap [1 ‐ 3]. Because of the constantly changing molecular arrangements, such correlations play a much more fundamental role in liquids than they do in other condensed phases such as glasses and solids, which sample only a few basic configurations. Indeed, a model with only repulsive intermolecular forces [2] can give a surprisingly accurate description of the full density correlations seen in a uniform dense simple liquid like Ar because the vector sum of the longer ranged attractive forces on a given particle essentially cancels [1] in most typical configurations. Nonuniform liquids present a greater and qualitatively different challenge, since even the averaged effects of attractive forces clearly do not cancel [4]. We discuss here an example where both attractive and repulsive forces can greatly influence the liquid’s structure: a LennardJones (LJ) fluid next to a hard wall. We obtain accurate numerical results using a physically suggestive generalized mean field description of the attractive forces [5]. We consider first the effects of these slowly varying forces on the liquid’s structure before taking account of the response to the rapidly varying (hard-core-like) part of the external field. This treatment of attractive interactions is quite different from that used in conventional integral equation and density functional methods [6], and we believe it offers important conceptual and computational advantages.