Influence of two-body and three-body interatomic forces on gas, liquid, and solid phases.

Accurate molecular dynamics simulations are reported which quantify the contributions of two- and three-body interactions in the gas, liquid, and solid phases of argon at both subcritical and supercritical conditions. The calculations use an accurate two-body potential in addition to contributions from three-body dispersion interactions from third-order triple-dipole interactions. The number dependence of three-body interactions is quantified, indicating that a system size of at least five hundred atoms is required for reliable calculations. The results indicate that, although the contribution of three-body interaction to the overall energy is small, three-body interactions significantly affect the pressure at which vapor-liquid and solid-liquid transitions are observed. In particular, three-body interactions substantially increase the pressure of the freezing point. Unlike two-body interactions, which vary with both density and temperature, for a given density, three-body interactions have a near-constant 'background' value irrespective of the temperature. Both two-body interactions and kinetic energy have an important role in vapor-liquid equilibria whereas solid-liquid equilibria are dominated by two-body interactions.