ABINITIO PSEUDOPOTENTIAL STUDY OF YB AND YBO

Large-scale self-consistent-field configuration interaction calculations with single and double excitations, multireference configuration interaction calculations, and coupled pair functional calculations using nonrelativistic and quasirelativistic energy-adjusted ab initio pseudopotentials for Yb, together with extended basis sets, have been performed for thirteen low-lying electronic states of YbO in the LAMBDA-S coupling scheme. Spin-orbit coupling was treated by means of energy-adjusted ab initio spin-orbit operators in connection with quasirelativistic double group configuration interaction calculations and configuration interaction calculations with iterative perturbative selection of the zeroth-order wave function for the twenty-five corresponding OMEGA states in the intermediate coupling scheme. Controversial assignments for the YbO ground-state electronic configuration derived from experimental facts interpreted within a ligand field model or previously published quasirelativistic pseudopotential calculations are discussed. Despite excellent results for YbH and YbF, the molecular constants of the lowest experimentally observed state of YbO (R(e) = 1.807 angstrom, D0 = 4.29 eV, omega(e) = 699 cm-1) recently assigned to be a 4f14-sigma-2-sigma-2-pi-4 1-SIGMA+ (OMEGA = 0+) ground state by McDonald et al. [J. Chem. Phys. 93, 7676 (1990)], show only limited agreement with the results obtained in the present work for this state (R(e) = 1.895 angstrom, D0 = 2.47 eV, omega(e) = 653 cm-1). In our most extensive calculations the lowest state arising from the 4f13-sigma-2-sigma-2-pi-4-sigma-1 configuration (OMEGA = 0-) is still found to be 0.93 eV lower in energy than the 4f14-sigma-2-sigma-2-pi-4 1-SIGMA+ (OMEGA = 0+) state. The importance of differential relativistic and correlation effects in the energy separation between states arising from configurations with differing 4f occupation is demonstrated and discussed.