Transition energies of barium and radium by the relativistic coupled-cluster method

The relativistic coupled-cluster method is used to calculate ionization potentials and excitation energies of the barium and radium atoms and their monocations. Large basis sets are used, with l up to 5, the Dirac-Fock or Dirac-Fock-Breit orbitals found, and the external 28 electrons of barium or 42 electrons of radium are correlated by the coupled-cluster method with single and double excitations. Good agreement (within a few hundred wave numbers) is obtained for the ionization potentials and low excitation energies (up to 3 eV for Ba, 4 eV for Ra). The Breit interaction has little effect on the excitation energies, but it improves significantly the fine-structure splittings of Ra. Large relativistic effects on the energies are observed, up to 1 eV for barium and 2 eV for radium. The nonrelativistic ground states of Ba⁺ and Ra⁺ are (n-1)d ²D rather than ns ²S. {} 1996 The American Physical Society.