Potential model for diatomic molecules including the united-atom limit and its use in a multiproperty fit for argon

Abstract

The extended Hartree–Fock approximate correlation energy potential model recently reported for diatomics has been improved to account for the Coulombic behaviour at the collapsed diatomic limit for vanishingly small interatomic separations, R→ 0. Also discussed is how to impose the appropriate inverse exponential dependence on R into the extended Hartree–Fock energy contribution as R→∞. Test results are presented for the ground states of 13 chemically stable diatomics and the Ar2 van der Waals molecule. For Ar2, the potential-energy function is obtained from a multiproperty fit including spectroscopic, scattering, and second virial coefficient data. For the remaining diatomics, the calibration procedure uses only spectroscopic Rydberg–Klein–Rees (RKR) data. The new Ar2 potential is then used to calculate the thermophysical properties of gaseous argon over the temperature range 102⩽T/K⩽ 105. In all cases, the potential-energy function of the present work has been found to be very accurate.