Accuracy of RCC-SD and PT2/CI methods in all-electron and RECP calculations on Pb and Pb2+

Transition energy calculations for low-lying states of Pb and Pb2+ by the f our-component versions of the Fock-space RCC-SD and PT2/CI methods are repo rted. 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 tes ted. The consideration of only one- and two-body amplitudes for Valence ele ctrons 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 est imated contributions of triple and quadruple excitations yields an accuracy of about 200 cm(-1).