TRANSITION ENERGIES OF BARIUM AND RADIUM BY THE RELATIVISTIC COUPLED-CLUSTER METHOD

The relativistic coupled-cluster method is used to calculate ionizatio n potentials and excitation energies of the barium and radium atoms an d their monocations. Large basis sets are used, with l up to 5, the Di rac-Fock or Dirac-Fock-Breit orbitals found, and the external 28 elect rons of barium or 42 electrons of radium are correlated by the coupled -cluster method with single and double excitations. Good agreement (wi thin a few hundred wave numbers) is obtained for the ionization potent ials 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 i t improves significantly the fine-structure splittings of Ra. Large re lativistic 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 D-2 rather than ns S-2.