MULTIPOLE POLARIZABILITIES OF THE HELIUM ATOM AND COLLISION-INDUCED POLARIZABILITIES OF PAIRS CONTAINING HE OR H-ATOMS

The transient, collision-induced changes in electric properties of ion -atom or atom-atom pairs at long range are determined by the polarizab ilities and hyperpolarizabilities of each of the interacting species. Induction effects on moments of the charge distribution or static susc eptibilities depend on the response at zero frequency, and dispersion effects depend on the polarizabilities (linear and nonlinear) at imagi nary frequencies. In this paper, we give numerical results for four st atic multipole hyperpolarizabilities of the helium atom obtained from large-basis, ab initio calculations using many-body perturbation theor y and coupled-cluster methods. We report and analyze the numerical res ults for the R(-6) and R(-8) terms in the dispersion contributions to the He-He, He-H, and H-H pair polarizability functions with a correcte d formula for the contributions form P-hyperpolarizability tensor. For both parallel and perpendicular components of the polarizability, the numerical results at order R(-8) have contributions of similar magnit ude arising from the terms containing the P and Q hyperpolarizabilitie s; these terms approximately cancel for Delta alpha(perpendicular to), but reinforce for Delta alpha(parallel to). When R is near to or smal ler than the van der Waals minimum distance, the dispersion series is damped by overlap, but the ratio of the undamped R(-6) and R(-8) terms gives a guide to the convergence behaviour. For the pairs treated her e, the R(-6) and R(-8) terms in Delta alpha(parallel to) become equall y important between 5 and 8 bohr, but the R(-6) term dominates the ser ies for Delta alpha(perpendicular to) down to separations of 3-4.5 boh r.