Excitation energies of Ba

Excitation energies of atomic barium and radium were calculated in 1996 using the Fock-space coupled cluster method [57]. The model space in the 2-electron sector included all states with two electrons in the 5d, 6s and 6p orbitals, except the 6p states inclusion of the latter led to intruder states and divergence, so that incomplete model spaces had to be employed. In the intermediate Hamiltonian approach all these states (including 6p ) were in Pm, Pi was defined by adding states with occupied 7s-10s, 7p-10p, 6d- 

Excitation energies of barium (cm ) by the Fock-space (FS) and intermediate Hamiltonian (IH) methods. Experiment from [59] 

Higher excitation energies in Ba, obtainable by the IH method only (cm ). Excited states belong to Pm or Pi. Experiment from [59]. 

and 4/-6/ orbitals, yielding a very large P = Pm + Pi) space [58]. This extended P led to much better accuracy for states which could be calculated by both methods (Table 4). The average error was reduced from 742 cm (relative error 3.11%) to 139 cm (relative error 0.69%). In addition, many more states, including those belonging to the 6p term, could be obtained (Table 5). The five Pm excitations in Table 5 show an average error of 415 cm or 1.2%, and the 19 Pi states deviate from experiment by 515 cm or 1.6%. This is unprecedented accuracy for systems of this size. Similar results were obtained for atomic radium [58]. 

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