Exploring the influence of three-body classical dispersion forces on phase equilibria of simple fluids: An integral-equation approach.

Abstract

We investigate a recently introduced integral equation which takes into account three-body interactions via an effective pair potential. The scheme proposed here essentially reduces to solving a reference hypernetted-chain equation with a state-dependent effective potential, and a hard-sphere reference bridge function that minimizes the free energy per particle. Our computational algorithm is shown to be stable and rapidly convergent. As a whole, the proposed procedure yields thermodynamic properties in accordance with simulation results for systems with Axilrod-Teller triple-dipole potential plus a Lennard-Jones interaction, and improves upon previous integral-equation calculations. Predictions obtained for the gas-liquid coexistence of argon are in remarkable agreement with experimental results, and show unequivocally that the influence of the three-body classical dispersion forces (and not only quantum effects) must be explicitly incorporated to account for the deviations between pure Lennard-Jones systems and real fluids. Moreover, the integral equation approach as introduced here proves to be a reliable tool and an inexpensive probe to assess the influence of three-body interactions in a consistent way. For completeness, the no-solution line of the integral equation is also presented. A study of the behavior of the isothermal compressibility in the vicinity of the no-solution boundary shows the presence of a divergence that deviates from a power law at high densities, and the appearance of a singularity with the characteristics of square-root branch point at low density (a feature also found in the hypernetted-chain approximation in a variety of systems). The no-solution line, unfortunately, hides the coexistence curve near the critical point, and this constitutes the only severe drawback in our approach. An alternative for bypassing this shortcoming is explored.