Abstract
Transition energy calculations for low-lying states of Pb and Pb2+ bythe four-component versions of the Fock-space RCC-SD and PT2/CImethods are reported. Contributions of valence and core electroncorrelation are studied in all-electron calculations with theDirac-Coulomb Hamiltonian. The accuracy of our generalized RECP and theRECP of Christiansen and co-workers is tested. The consideration of onlyone- and two-body amplitudes for valence electrons in the RCC method forPb is shown not to be sufficient to reproduce valence excitationswithin 100-300 cm-1. Correcting RCC-SD results by estimatedcontributions of triple and quadruple excitations yields an accuracy ofabout 200 cm-1.
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