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.
Published Version
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