Actinyl ions in Cs2UO2Cl4

The electronic energy levels of the uranyl ion (UO22+) and the neptunyl ion (NpO22+) in the crystalline environment of Cs2UO2Cl4 are studied theoretic ally and compared with the spectroscopic work of Denning and co-workers, A layered-cluster computational method is used. The valence electrons of the actinyl ion and the nearest-neighbor chloride ions are treated explicitly, the closest cesium ions are replaced by all-electron core potentials, and a ll ions further away are replaced by point charges. The cluster is approxim ately spherical overall and contains 1873 ions. For the electrons treated e xplicitly, we use relativistic quantum chemical theory, including relativis tic effective core potentials, corresponding spin-orbit operators, and spin -orbit, graphical unitary group configuration interaction. The effects of t he crystalline environment on bond distances, vibrational frequencies, exci tation energies, energy splittings, and wave function character are examine d. Shifts are generally more accurate than absolute values, and the electro n correlation treatment is generally the limiting factor in the accuracy.