Vapor-Pressure Test for Intermolecular Potentials

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Abstract
Radial distribution functions g(r) obtained from x-ray and neutron diffraction measurements are used to calculate the vapor pressure of liquid Ar for various empirical potential functions u(r) with parameters chosen to fit gas data. The potentials investigated were the (12,6), (exp-6), Kihara, Kingston, Barker–Pompe, and Dymond–Alder. The results are sensitive to the choice of g(r) and of u(r) and to the potential parameters used with u(r); they are highly sensitive to the threshold behavior of g(r) and also to the slow-collision diameter σ of u(r). The nonadditivity correction to the pressure was calculated from the triple-dipole potential using the superposition approximation. Results are in error by varying orders of magnitude, the (12,6) potential giving the largest disagreement with experiment. The Kingston potential and the Kihara potential with Barker's parameters are consistent with experiment considering the large uncertainties (∼ 300 atm) of the method. The Barker–Pompe and Dymond–Alder potentials give the best vapor pressures, well within these uncertainties. The results suggest that the true value of σ is close to 3.3 Å. The threshold region of g(r) requires more careful experimental investigation.