EXPLORING THE INFLUENCE OF 3-BODY CLASSICAL DISPERSION FORCES ON PHASE-EQUILIBRIA OF SIMPLE FLUIDS - AN INTEGRAL-EQUATION APPROACH

We investigate a recently introduced integral equation which takes int o account three-body interactions via an effective pair potential. The scheme proposed here essentially reduces to solving a reference hyper netted-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 r apidly convergent. As a whole, the proposed procedure yields thermodyn amic properties in accordance with simulation results for systems with Axilrod-Teller triple-dipole potential plus a Lennard-Jones interacti on, and improves upon previous integral-equation calculations. Predict ions obtained for the gas-liquid coexistence of argon are in remarkabl e 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 d eviations between pure Lennard-Jones systems and real fluids. Moreover , the integral equation approach as introduced here proves to be a rel iable tool and an inexpensive probe to assess the influence of three-b ody interactions in a consistent way. For completeness, the no-solutio n line of the integral equation is also presented. A study of the beha vior of the isothermal compressibility in the vicinity of the no-solut ion boundary shows the presence of a divergence that deviates from a p ower law at high densities, and the appearance of a singularity with t he characteristics of square-root branch point at low density (a featu re also found in the hypernetted-chain approximation in a variety of s ystems). The no-solution line, unfortunately hides the coexistence cur ve near the critical point, and this constitutes the only severe drawb ack in our approach. An alternative for bypassing this shortcoming is explored.