ON THE USE OF SEMIPHENOMENOLOGICAL CLOSURES IN INTEGRAL-EQUATIONS FOR CLASSICAL FLUIDS

We have studied two choices of semiphenomenological closures for the Ornstein-Zernike equation, both for a monoatomic Lennard-Jones fluid and a dipolar homonuclear hard diatomic fluid. One of the closures was originally proposed by Verlet for hard-sphere systems, for which is known to yield good results. A second closure is proposed by us in the frame of the reference hypernetted chain (RHNC) theory. We have described the reference systems in this closure by means of Verlet's approximation and its recent extension to systems of nonspherical particles. This second approach, which we denote by RHNC-VM (Verlet's modified), turns out to give an excellent description of the structure and thermodynamics of the Lennard-Jones fluid and very accurate predictions for the structure of the dipolar diatomic system. In this latter case the apparent superiority of hypernetted chain results for configurational energies is found to stem from fortuitous cancellation of errors in the integration of the components of the pair correlation function. Nonetheless, an approximation for the bridge function capable of accounting for the particular dielectric behavior of the dipolar diatomic fluid is still lacking.