Interatomic interactions and the Cotton-Mouton effect for helium.

We report calculations of the interaction-induced polarizability ($\Delta \alpha^{anis}$), magnetizability ($\Delta \xi^{anis}$) and hypermagnetizability ($\Delta \eta^{anis}$) anisotropies for the helium gas as a function of the interatomic separation. From these data we determine the virial coefficients for the Cotton--Mouton effect and the hypermagnetizability anisotropy of helium. We also find the mean polarizability and magnetizability as a function of the interatomic separation and the virial coefficients for these properties. The results for the Cotton--Mouton effect indicate that pressure affects the Cotton--Mouton constant to the same extent as it does the second hyperpolarizability ($\gamma$) and the virial coefficient $b_{CME}(\omega,T)$ lies in the range of $-1.6$ to $-1.8$ cm$^3$ mol$^{-1}$. This means that pressure effects for the Cotton--Mouton constant could be detected with modern experimental techniques. All calculations were carried out using the full configuration interaction technique and large basis sets of London atomic orbitals. The polarizability calculations were performed both for relevant optical frequencies as well as the static case.