- An expression for the frequency dependent polarizabiHty in the multiconfigurational time dependent Hartree-Fock (MCTDHF) approximation is derived. MCTDHF calculations are performed on O2 and Be. The basis set dependence of the calculated O2 polarizabilities is examined, and comparison is made with the experiments and with the polarizabilities of simplified approximations. These results indicate that the experimental frequency dependent polarizability can be reproduced most consistently in the MCTDHF approximation. 1980 Taylor and Francis Ltd.