Abstract :
Rydberg basis set functions were used in ab initio coupled perturbed Hartree-Fock calculations of second hyperpolarizabilities γ of H2, C6H6 and CS2 in order to investigate their effects on γ. In all the cases, Rydberg functions were found to influence γ remarkably. The H2 results were reasonable as compared with the exact James-Coolidge (J-C) calculation of Bishop et al. [19], even when only the lowest 2sp Rydberg functions were used. However, the C6H6 and CS2 calculations showed smaller 〈γ〉 than EFISH experiments in the liquid state, since most of the experimental 〈γ〉 comes from the vibrational and rotational motion of molecules. The results indicated that the Rydberg character of excited states has a significant effect on γ, in agreement with the Ward formula which regards excited states as the source of γ. The role of diffuse functions which were used in previous ab initio studies was discussed, considering that they can also serve to describe the Rydberg character of excited states.
