Accuracy of RCC-SD and PT2/CI methods in all-electron and RECP calculations on Pb and Pb2+
T A Isaev1, N S Mosyagin1, M G Kozlov1,
A V Titov1, E Eliav2 and U Kaldor2
1 Petersburg Nuclear Physics Institute, Gatchina, St
Petersburg district 188350, Russia
2 School of Chemistry, Tel Aviv University, Tel Aviv
69978, Israel
Transition energy calculations for low-lying states of Pb and Pb2+ by the four-component versions of the Fock-space RCC-SD and PT2/CI methods are reported. Contributions of valence and core electron correlation are studied in all-electron calculations with the Dirac-Coulomb Hamiltonian. The accuracy of our generalized RECP and the RECP of Christiansen and co-workers is tested. The consideration of only one- and two-body amplitudes for valence electrons in the RCC method for Pb is shown not to be sufficient to reproduce valence excitations within 100-300 cm-1. Correcting RCC-SD results by estimated contributions of triple and quadruple excitations yields an accuracy of about 200 cm-1.