On the electronic structure of the UO2 molecule

The structure and vibrational frequencies of the UO2 molecule have been determined using multiconfigurational wave functions (CASSCF/CASPT2), together with a newly developed method to treat spin-orbit coupling. The molecule has been found to have a (5f phi)(7s), (3)Phi (u), Omega = 2 ground state with a U-O bond distance of 1.77 Angstrom. The computed antisymmetric stretching a. frequency is 923 cm(-1) with a 16/18 isotope ratio of 1.0525 which compares with the experimental values of 915 cm(-1) and 1.0526, respectively. Calculations of the first adiabatic ionization energy gave the value 6.17 eV, which is 0.7 eV larger than the currently accepted experimental result. Reasons for this difference are suggested.