Triple-Dipole Potentials in Classical Nonpolar Fluids

Formulas which express the triple-dipole contribution to the pressure, internal energy, effective pair potential, and density expansion of the radial distribution function appropriate to classical nonpolar fluids are evaluated. A rational approximation to tabulated theoretical values of the radial distribution function for argon based on the Lennard-Jones (12, 6) model is used to facilitate the numerical evaluation of the two- and three-fold integrals involved. The nonadditive contribution to the pressure and internal energy of argon is several percent over the temperature-density range considered which includes the critical region. It is found that the triple-dipole energy cannot account for the observed variation with density of the effective pair potential for argon reported by Mikolaj and Pings.