Comparison of relativistic effective core potential and all-electron Dirac-Coulomb calculations of mercury transition energies by the relativistic coupled-cluster method

Relativistic effective core potential (RECP) and all-electron Dirac-Coulomb calculations of transition energies for low-lying states of the mercury at om and its ions are carried out with equivalent basis sets by the relativis tic coupled-cluster method. RECP results are compared with corresponding al l-electron values to estimate the accuracy of different RECP versions. Cont ributions from correlations of different electron shells to the calculated transition energies are studied. Effects of different nuclear models and of basis set truncation at different orbital angular momenta as well as error s of the Gaussian approximation of the generalized RECP components are repo rted. We show that at least 34 external electrons of the mercury atom shoul d be correlated and the one-electron basis set should contain up to h-type functions in order to attain consistent agreement within 200 cm(-1) with ex perimental data.