Vibrational corrections to electric properties of weakly bound systems

The pure electronic and vibrational contributions to electric dipole moment s, dipole polarizabilities, and first hyperpolarizabilities have been evalu ated for the HF and H2O dimers. The zero-point vibrational average correcti ons to dipole moments and dipole polarizabilities turn out to be relatively small. However, the corresponding contributions to the first hyperpolariza bility are found to be of the same magnitude as the pure electronic values. The so-called pure vibrational corrections to the dipole polarizability an d first hyperpolarizability of hydrogen bonded dimers are exceptionally lar ge and indicate that the perturbation theory method used for their evaluati on fails to account properly for the high mechanical and electric anharmoni cities present in these systems. The analysis of different harmonic and anh armonic contributions to the pure vibrational correction to the first hyper polarizability shows explicitly the importance of the low frequency intermo lecular modes.