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An implementation of the relativistic multireference Fock-space coupled cluster method is presented which allows simultaneous calculation of potential surfaces for different oxidation states and electronic levels of a molecule, yielding values for spectroscopic constants and transition energies. The method is tested in pilot calculations on the I 2 and HgH molecules, and is shown to give a good and balanced description of various electronic states and energies.

The ground and excited states of the UO 2 molecule have been studied using a Dirac-Coulomb intermediate Hamiltonian Fock-space coupled cluster approach ͑DC-IHFSCC͒. 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 UO 2 + and UO 2 2+ as well as on the ions U 4+ and U 5+ 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 UO 2 molecule as a 3 ⌽ 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 .

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