Ab initio calculations of the vibrational contributions to the static dipole polarizability of CO2 and CS2

Assuming that the Born-Oppenheimer approximation is valid, the electronic and vibrational contributions to the static dipole polarizability of CO2 and CS2 have been computed at the MP2/6-311 + + G(3df,3pd) level. An adapted clamped-nucleus method was used: the nuclear conformation was optimized without external field, then with a variety of applied fields F, parallel or perpendicular to the molecular axis. The corresponding harmonic vibrational frequencies were computed to obtain the zero-point vibrational energy. Finite-field calculations, based on central difference formulae, were developed in three steps, to obtain the pure electronic values of the parallel and perpendicular diagonal components of the polarizability tensor, then the concomitant vibrational contributions. Comparisons with experimental results and previous calculations by the sum-over-states method are reported. (C) 1997 Elsevier Science B.V.