A Fock space coupled cluster study on the electronic structure of the UO2, UO2+, U4+, and U5+ species

The ground and excited states of the UO2 molecule have been studied using a Dirac-Coulomb intermediate Hamiltonian Fock-space coupled cluster approach (DC-IHFSCC). This method is unique in describing dynamic and nondynamic correlation energies at relatively low computational cost. Spin-orbit coupling effects have been fully included by utilizing the four-component Dirac-Coulomb Hamiltonian from the outset. Complementary calculations on the ionized systems UO2+ and UO22+ as well as on the ions U4+ and U5+ were performed to assess the accuracy of this method. The latter calculations improve upon previously published theoretical work. Our calculations confirm the assignment of the ground state of the UO2 molecule as a (3)Phi(2u) state that arises from the 5f(1)7s(1) configuration. The first state from the 5f(2) configuration is found above 10 000 cm(-1), whereas the first state from the 5f(1)6d(1) configuration is found at 5 047 cm(-1). (C) 2007 American Institute of Physics.