A comparison of the predictions of various model N2–He potential energy surfaces with experiment

Accuracy of recent potential energy surfaces for the He–N2 interaction. I. Virial and bulk transport coefficients

A new exchange-Coulomb semiempirical model potential energy surface for the He-N2 interaction has been developed. Together with two recent high-level ab initio potential energy surfaces, it has been tested for the reliability of its predictions of second-virial coefficients and bulk transport phenomena in binary mixtures of He and N2. The agreement with the relevant available measurements is generally within experimental uncertainty for the exchange-Coulomb surface and the ab initio surface of Patel et al. [J. Chem. Phys. 119, 909 (2003)], but with slightly poorer agreement for the earlier ab initio surface of Hu and Thakkar [J. Chem. Phys. 104, 2541 (1996)].

A new approximation for atom-diatom rotational-relaxation cross sections

A semiclassical approximation to the S matrix of the infinite-order-sudden approximation is introduced. This is employed to yield for the energy-transfer effective cross section a purely classical approximation, analogous to the Mason-Monchick approximation [J. Chem. Phys. 36, 1622 (1962)] for traditional collision integrals. Constraints on energy and on angular momentum transfer are included. Numerical evaluation of this new approximation can readily be performed alongside that for traditional collision integrals. The new result is tested against full classical trajectory calculations for six potential energy surfaces for the collision systems H-N(2), He-N(2), He-CO, and Ar-CO(2). Differences of no more than 15% from the classical trajectory calculations have been obtained.