Multi-property predictions from recent He-CO potential energy surfaces and related comments on the nature of heteronuclear rare gas interactions

Recently two potential energy surfaces, an empirical potential V-(3,V-3,V-3) and a potential of the exchange-Coulomb (XC) form, have been determined by fitting their adjustable parameters to the observed line positions of the same high resolution infra-red spectra of He-CO. Both yield results for the IR spectra in excellent agreement with experiment and with each other but, interestingly, there are significant differences between the XC and V-(3,V-3,V-3) potentials as a function of relative orientation and interspecies distance for all regions of configuration space, and not just for the repulsive walls. In this paper several properties of the He-CO dimer, second virial, binary diffusion and shear viscosity (both interaction and mixture) coefficients, are used to discriminate between XC, V-(3,V-3,V-3) and several other literature potentials. The explicit calculations of the transport properties are carried out in the Mason-Monchik approximation and then scaled to give good estimates of close coupling/classical trajectory results. Comparison with experiment indicates that the XC potential is currently the preferred potential energy surface for He-CO. It is emphasized also that the properties obtained from the XC and V-(3,V-3,V-3) surfaces are similar, much more similar than a comparison of the potentials would indicate. The reasons for this are discussed, using in part comparisons of the cross-sections relevant to viscomagnetic effects and to the pressure broadening of depolarized Rayleigh light scattering, which were calculated for XC and V-(3,V-3,V-3). The effects responsible will not occur for the interaction of a rare gas with a homonuclear diatomic molecule and illustrate the difficulty inherent in determining the relative reliability of potential energy functions for interactions involving heteronuclear diatomic molecules.