Theoretical Study of the van der Waals Forces in Alkali-Noble-Gas Systems

The long-range interaction in alkali-noble-gas systems is studied in the multiconfiguration-self-consistent-field (MCSCF) formalism. Owing to the diffuse valence shell of the alkali, there are important contributions to the interaction potential from the higher-order multipole moments of the alkali (although the dipole moment states are by far the most important). The MCSCF formalism is shown to lead to a simple and effective way of handling the above effect, as well as evaluating intra-atomic correlation contributions and the coupling between interatomic and intra-atomic correlations when they are important. The nature of the intra-atomic-interatomic correlation coupling is analyzed and the relationship of the MCSCF results to perturbation-theory considerations clarified. Computed interaction potentials are presented for the systems HeH and LiHe. The results for HeH are in good agreement with the corresponding experimental results, while those for the system LiHe, agreeing with Dalgarno's recommended value of $C_6$, differ markedly from Baylis's semiempirical calculations.