Triple-dipole Potential Research Articles

Vapor-Pressure Test for Intermolecular Potentials

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.

Triple-dipole dispersion forces in dense fluids

The contribution of triple-dipole dispersion forces to thermodynamic properties of fluid argon is determined from an analysis of computer-generated configurations of atoms. For states of liquid argon lying near the liquid-vapour equilibrium line the effect of triple-dipole interactions is to increase both internal energy and volume by several per cent. The inclusion of the triple- dipole contribution largely accounts for the discrepancies which are found between the experimental values of internal energy and pressure and the values calculated from a potential chosen to simulate the true pair potential between argon atoms. Accurate values are also obtained for the contribution of the triple-dipole potential to medium-dependent effective pair potentials of the type developed by Sinanoglu (1967). Results are given in tabular form for six states of fluid argon and for the corresponding states of krypton and xenon.