On the effects of spin-orbit coupling on molecular properties:: Dipole moment and polarizability of PbO and spectroscopic constants for the ground and excited states

The lower electronic states of the molecule PbO have been studied using a newly developed basis set of the atomic natural orbital (ANO) type. The method includes scalar relativistic effects through the use of a Douglas-Kroll Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second-order perturbation theory (CASSCF/CASPT2). Spin-orbit coupling is added a posteriori by means of the RASSCF state interaction (RASSI) method, with all electronic states originating from Pb(s(2)p(2),P-3, D-1, S-1) and O(s(2)p(4), (3)p) included in the Cl expansion. Computed spectroscopic constants for the 0(+) ground level are: R-e = 1.926(1.922)angstrom, D-0 = 4.00(3.83) eV, omega(e), = 723(721) cm(-1), with experimental data within parentheses. Corresponding data for the 11 lowest excited levels are also presented. The dipole moment for the ground state has been computed to be 1.73(1.83) a.u. using finite field perturbation theory. The effect of spin-orbit coupling is to reduce the value with 0.03 a.u. This illustrative calculation shows that an approach that adds scalar relativistic effects to a nonrelativistic wave function, and treats spin-orbit coupling by a configuration interaction method, can be used to obtain accurate properties also for molecules containing heavy main group elements.