On the electronic structure of the UO2 molecule

The structure and vibrational frequencies of the UO2 molecule have been det ermined using multiconfigurational wave functions (CASSCF/CASPT2), together with a newly developed method to treat spin-orbit coupling. The molecule h as been found to have a (5f phi)(7s), (3)Phi (u), Omega = 2 ground state wi th a U-O bond distance of 1.77 Angstrom. The computed antisymmetric stretch ing a. frequency is 923 cm(-1) with a 16/18 isotope ratio of 1.0525 which c ompares 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.