Combining intermolecular potentials for the prediction of fluid properties: Two-body and three-body interactions.

Abstract

The ability to combine intermolecular potentials without loss of information is investigated. Molecular simulation results for both vapor-liquid equilibria and supercritical isochoric heat capacities are reported for different combinations of n-m potentials. The role of both additional cohesion and repulsive terms is determined. The 12-8-6 potential obtained by adding an m = 6 contribution to the 12-8 potential significantly broadens the phase envelope, which remains inside of the 12-6 envelope. In contrast, the 12+9-6 potential that involves an additional n = 9 repulsive contribution lifts the phase envelope above the 12-6 values. The 12-8-6 potential significantly reduces the maximum and minimum observed for the isochoric heat capacity at supercritical conditions. In contrast, the additional repulsion of the 12+9-6 potential has a relatively small influence on the supercritical behavior of the isochoric heat capacity. Significantly, a comparison of vapor-liquid equilibria data for two-body only simulations for Ar, Kr, and Xe indicates that there is very good agreement with the 12-8-6 data. This means that the 12-8-6 potential may provide a useful description of two-body only interactions for the noble gases. The 12+9-8 potential at least partially reproduces vapor-liquid properties of noble gases interacting via two-body plus three-body interactions. In general, the combination of potentials provides a mechanism of simplifying the calculation of two-body and two-body plus three-body interactions.