QUASI-RELATIVISTIC STUDIES OF MOLECULAR ELECTRIC PROPERTIES - DIPOLE-MOMENTS OF THE GROUP-IVA OXIDES AND SULFIDES

Dipole moments of SiO, GeO, SnO, PbO, and of the corresponding sulfieds are computed by using a variety of high-level correlated methods and polarized basis sets of Gaussian functions. The contribution of relativistic effects has been accounted for by the first-order quasirelativistic approach based on mass-velocity and Darwin terms. The results for sulfides are in good agreement with experiment while for oxides seem to show a progressive deterioration with the increase of the heavy atom nuclear charge. The discrepancies between the calculated and experimental dipole moments for oxides have been carefully analyzed for the PbO molecule in terms of the basis set choice and reliability of the calculated electron correlation and relativistic corrections. The pattern of relativistic corrections computed in the present study is compared with that which follows from recent quasirelativistic and fully relativistic calculations of Dyall. Some complementary calculations of the dipole moment of PbSe and PbTe seem to indicate that both the polarized basis sets and computational methods used in this study should not fail to predict correct values of the dipole moment of PbO and other oxides of the group IVa atoms. The rather unexpected discrepancies have no plausible explanation within the range of methods employed in our study and offer a challenge for more advanced, fully relativistic molecular calculations at the correlated level of approximation.