A self-consistent integral equation study of the structure and thermodynamics of the penetrable sphere fluid

The penetrable sphere fluid consists of a system of spherical particles int eracting via a potential that remains finite and constant for distances sma ller than the particle diameter and is zero otherwise. This system, which w as proposed sometime ago as a model for micelles in a solvent, has represen ted so far a remarkable challenge for integral equation theories which prov ed unable to correctly model the behavior of the two-body correlations insi de the particle overlap region. It is shown in this work that enforcing the fulfillment of zero separation theorems for the cavity distribution functi on y(r), and thermodynamic consistency conditions (fluctuation vs virial co mpressibility and Gibbs-Duhem relation), on a parametrized closure of the t ype proposed by Verlet, leads to an excellent agreement with simulation, bo th for the thermodynamics and the structure (inside and outside the particl e core). Additionally, the behavior of the integral equation at high packin g fractions is explored and the bridge functions extracted from simulation are compared with the predictions of the proposed integral equation. (C) 20 00 American Institute of Physics. [S0021-9606(00)51102-7].