We present a 4-component relativistic study of uranium 2p3/2 ionization and excitation in the isoelectronic series UO2(2+), OUN(+) and UN2. We calculate ionization energies by ΔSCF at the Hartree-Fock (HF) and Kohn-Sham (KS) level of theory. At the ΔHF level we observe a perfectly linear chemical shift of ionization energies with respect to uranium atomic charges obtained from projection analysis. We have also developed a non-canonical 2nd-order Møller-Plesset code for wave function based correlation studies. We observe the well-known failure of Koopmans' theorem for core ionization due to the dominance of orbital relaxation over electron correlation effects. More unexpectedly, we find that the correlation contribution has the same sign as the relaxation contribution and show that this is due to a strong coupling of relaxation and correlation. We simulate uranium L3 XANES spectra, dominated by 2p3/2 → U6d transitions, by restricted excitation window time-dependent density functional theory (REW-TDDFT) and the complex polarization propagator (CPP) approach and demonstrate that they give identical spectra when the same Lorentz broadening is chosen. We also simulate XANES spectra by the Hartree-Fock based static exchange (STEX) method and show how STEX excitation energies can be reproduced by time-dependent Hartree-Fock calculations within the Tamm-Dancoff approximation. We furthermore show that Koopmans' theorem provide a correct approximation of ionization energies in the linear response regime and use this observation to align REW-TDDFT and CPP spectra with STEX ones. We point out that the STEX method affords the most detailed assignment of spectra since it employs virtual orbitals optimized for the selected core ionization. The calculated XANES spectra reflect the loss of bound virtual orbitals as the molecular charge is reduced along the isoelectronic series.

Neodymium monofluoride dication was studied as a model of the Nd-F bond in NdFx. Multiconfigurational self-consistent field (MCSCF) and second order multireference quasi-degenerate perturbation theory (MCQDPT2) methods were used with a variety of active spaces to elucidate the roles of the Nd 4f, 5d, and 6s orbitals. Spin-orbit coupling calculations were performed at the SO-MCQDPT2 level, and potential energy curves were obtained for the four lowest energy quartet states as well as for the four lowest doublet states and the lowest sextet state. Inclusion of spin-orbit coupling splits these states into 30 levels. Equilibrium bond lengths, dissociation energies, transition energies, and crossing points were determined.

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