Intermolecular forces and the structure of uniform and nonuniform fluids

We discuss the ramifications of Widom's idea that attractive intermolecular forces essentially cancel in dense uniform liquids. This idea was used directly in the WCA theory of uniform liquids, where the structure of the liquid is approximated by that of a simpler reference fluid with purely repulsive intermolecular forces. To take account of the unbalanced attractive forces found in nonuniform fluids, Weeks, Selinger, and Broughton (WSB) developed a new method where the structure is related to that of a nonuniform reference fluid in an external field chosen to yield a self-consistent description of correlations induced by the repulsive forces and a mean field treatment of the attractive forces. Using simulations, we provide a quantitative test of the accuracy of both methods for the uniform fluid at different points in the phase diagram by relating correlation functions in the uniform fluid to those in a nonuniform fluid with a particle fixed at the origin. We find that at high densities the WSB approach can correct most of the small errors in the structure of the WCA reference fluid. At lower densities the WSB method provides a considerable improvement over the WCA theory. A simplified version of the WSB method is presented that is of comparable accuracy.