Excited States and Absorption Spectra of UF6: A RASPT2 Theoretical Study with Spin-Orbit Coupling.

Uranium hexafluoride (UF6) is an important compound in nuclear chemistry. The theoretical investigation of its excited states is difficult due to the large number of uranium valence orbitals and ligand lone pairs. We report here a detailed relativistic quantum chemical investigation of its excited states up to about 10 eV using restricted active space second-order perturbation theory (RASPT2). Scalar and spin-orbit (SO) relativistic effects are treated by a relativistic small-core pseudopotential. The RASPT2/SO results remain moderately accurate when the electrons in the active space are restricted to single and double excitations. All eight major spectral peaks corresponding to ligand-to-metal charge transfer have been reproduced within an accuracy of about 0.2 eV and are tentatively assigned. We find that BLYP-based hybrid density functional with 35% Hartree-Fock exchange well reproduce the excitation energies of UF6.