TIME-DEPENDENT HARTREE-FOCK 2ND-ORDER MOLECULAR-PROPERTIES WITH A MODERATELY SIZED BASIS SET .1. THE FREQUENCY-DEPENDENCE OF THE DIPOLE POLARIZABILITY

Time-dependent coupled Hartree-Fock calculations of the frequency-dependent mean polarizability alpha-(omega) and its anisotropy DELTA-alpha-(omega) 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 alpha-(omega), are systematically underestimated by 20% and 40%, respectively, with the 6-31G(+ sd + sp) basis set. The dispersion in DELTA-alpha-(omega) 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 DELTA-alpha-(omega) by a greater amount. This seems to be the result of systematic experimental overestimates of dispersion in DELTA-alpha-(omega) for molecules with very small Rayleigh depolarization ratios (rho-0 < 10(-2)), suggesting that the vibrational Raman contribution in these experiments is not only important, but significantly frequency dependent.