Spin-orbit configuration interaction study of the electronic structure of the 5f2 manifold of U4+ and the 5f manifold of U5+

10 pages; International audience; Cécile Danilo,1,2 Valérie Vallet,1 Jean-Pierre Flament,1 and Ulf Wahlgren2,3 1Université de Sciences et Technologies de Lille 1, Laboratoire PhLAM, CNRS UMR 8523, CERLA, CNRS FR 2416, Bât. P5, F-59655 Villeneuve d'Ascq Cedex, France 2Department of Physics, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden 3NORDITA, AlbaNova University Centre, 106 91 Stockholm, Sweden The energy levels of the 5f configuration of U5+ and 5f2 configuration of U4+ have been calculated in a dressed effective Hamiltonian relativistic spin-orbit configuration interaction framework. Electron correlation is treated in the scalar relativistic scheme with either the multistate multireference second-order multiconfigurational perturbation theory (MS-CASPT2) or with the multireference single and double configuration interaction (MRCI) and its size-extensive Davidson corrected variant. The CASPT2 method yields relative energies which are lower than those obtained with the MRCI method, the differences being the largest for the highest state 1S0 of the 5f2 manifold. Both valence correlation effects and spin-orbit polarization of the outer-core orbitals are shown to be important. The satisfactory agreement of the results with experiments and four-component correlated calculations illustrates the relevance of dressed spin-orbit configuration interaction methods for spectroscopy studies of heavy elements.