The interaction polarizability and interaction second-hyperpolarizability for He center dot center dot center dot He

Calculations are reported of the mean polarizability Delta<(alpha)over bar> and the mean second hyperpolarizability Delta<(gamma)over bar> which emanate from the interaction between two helium atoms. The results are based on three levels of calculation (SCF, MP2 and MP4) and for a wide range of interparticle separations. The values of Delta<(alpha)over bar> and Delta<(gamma)over bar>, after correction for basis set superposition error, are used (a) to discuss long-range dispersion and dipole-induced-dipole effects, and (b) to assess the approximate formulas for these effects which have been obtained from perturbation theory as opposed to the present 'supermolecular' calculations. The results are used also to determine the second dielectric virial coefficient and the second virial coefficient of the second hyperpolarizability; these are found to be -0.0614 cm(6) mol(-2) and -1.41 cm(3) mol(-1), respectively.