Geometries, electronic structures, and excited states of UN2, NUO~+, and UO2~(2+): a combined CCSD(T), RAS/CASPT2 and TDDFT study

摘要

The ground- and excited-state geometries and electronic structures of the isoelectronic series of molecules UN2, NUO~+, and UO2~(2+) are investigated by using relativistic density functional theory (DFT) and ab initio wavefunction theory (WFT). Scalar relativistic and spin-orbit-coupled quantum chemical methods at the CASPT2, RASPT2, CCSD(T), DFT and TDDFT levels are applied. Relativistic effects as elucidated by Pekka Pyykko play an important role in these uranium compounds, in particular for the excited states. The three molecular species exhibit significantly different spectroscopic properties, concerning their excitation energies, bond lengths and vibrations. Density functional approaches yield qualitatively correct results for the ground states and the valence → U.7s,6d excited states. However, the performance of TDDFT for valence → U.5f type excitations (in particular of UN2 and NUO~+) is less satisfactory, indicating the importance of the self-interaction correction for such excitations.