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 U O2 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 U O2+ and U O2 2+ 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 U O2 molecule as a 2u 3 state that arises from the 5 f1 7 s1 configuration. The first state from the 5 f2 configuration is found above 10 000 cm-1, whereas the first state from the 5 f1 6 d1 configuration is found at 5 047 cm-1.