Time-dependent Hartree–Fock second-order molecular properties with a moderately sized basis set. I. The frequency dependence of the dipole polarizability

Time-dependent coupled Hartree–Fock calculations of the frequency-dependent mean polarizability α(ω) and its anisotropy Δα(ω) are presented for a wide range of molecules containing first and second row atoms. The Cauchy moments S(−2) and S(−4), which describe the dispersion in α(ω), are systematically underestimated by 20% and 40%, respectively, with the 6-31G(+sd+sp) basis set. The dispersion in Δα(ω) is also underestimated, typically by 50%. Many exceptions to this behavior are observed, most of which appear to be an underestimate of the experimental dispersion in Δα(ω) by a greater amount. This seems to be the result of systematic experimental overestimates of dispersion in Δα(ω) for molecules with very small Rayleigh depolarization ratios (ρ0<10−2), suggesting that the vibrational Raman contribution in these experiments is not only important, but significantly frequency dependent.