Theoretical investigation of the energies and geometries of photoexcited uranyl(Vl) ion:A comparison between wave-function theory and density functional theory

摘要

In order to assess the accuracy of wave-function and density functional theory (DFT) based methods for excited states of the uranyl(VI) UO_2~(2+) molecule excitation energies and geometries of states originating from excitation from the sigma_u,sigma_g,pi_U,and pi_g orbitals to the nonbonding 5f_(delta) and 5f_(phi) have been calculated with different methods.The investigation included linear-response CCSD (LR-CCSD),multiconfigurational perturbation theory (CASSCF/CASPT2),size-extensivity corrected multireference configuration interaction (MRCI) and AQCC,and the DFT based methods time-dependent density functional theory (TD-DFT) with different functionals and the hybrid DFT/ MRCI method.Excellent agreement between all nonperturbative wave-function based methods was obtained.CASPT2 does not give energies in agreement with the nonperturbative wave-function based methods,and neither does TD-DFT,in particular,for the higher excitations.The CAM-B3LYP functional,which has a corrected asymptotic behavior,improves the accuracy especially in the higher region of the electronic spectrum.The hybrid DFT/MRCI method performs better than TD-DFT,again compared to the nonperturbative wave-function based results.However,TD-DFT,with common functionals such as B3LYP,yields acceptable geometries and relaxation energies for all excited states compared to LR-CCSD.The structure of excited states corresponding to excitation out of the highest occupied sigma_u orbital are symmetric while that arising from excitations out of the pi_U orbitals have asymmetric structures.The distant oxygen atom acquires a radical character and likely becomes a strong proton acceptor.These electronic states may play an important role in photoinduced proton exchange with a water molecule of the aqueous environment.