Ab initio pseudopotential study of Yb and YbO

Abstract

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 ΛS 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 Ω 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 (Re = 1.807 Å, D0 = 4.29 eV, ωe = 699 cm−1) recently assigned to be a 4f14σ 2σ 2π4 1Σ+ (Ω = 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 (Re= 1.895 Å, D0 = 2.47 eV, ωe = 653 cm−1). In our most extensive calculations the lowest state arising from the 4f13σ 2σ 2π4σ 1 configuration (Ω = 0−) is still found to be 0.93 eV lower in energy than the 4f14σ 2σ 2π4 1Σ+ (Ω = 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.