Comparative electronic structure of a lanthanide and actinide diatomic oxide: Nd versus U

Abstract

Using a modified version of the Alchemy electronic structure code and relativistic pseudopotentials, the electronic structure of the ground and low lying excited states of UO, NdO, and NdO⁺ have been calculated at the Hartree—Fock (HF) and multiconfiguration self-consistent field (MCSCF) levels of theory. Including results from an earlier study of UO⁺ this provides the information for a comparative analysis of a lanthanide and an actinide diatomic oxide. UO and NdO are both described formally as M⁺²O⁻² and the cations as M⁺³O⁻², but the HF and MCSCF calculations show that these systems are considerably less ionic due to large charge back-transfer in the π orbitals. The electronic states putatively arise from the ligand field (oxygen anion) perturbed f⁴, sf³, df³, sdf², or s²f² states of M⁺² and f³, sf² or df² states of M⁺³. Molecular orbital results show a substantial stabilization of the sf³ or s²f² configurations relative to the f⁴ or df³ configurations that are the even or odd parity ground states in the M⁺² free ion. The compact f and d orbitals are more destabilized by the anion field than the diffuse s orbital. The ground states of the neutral species are dominated by orbitals arising from the M⁺²sf³ term, and all the potential energy curves arising from this configuration are similar, which allows an estimate of the vibrational frequencies for UO and NdO of 862 cm⁻¹ and 836 cm⁻¹, respectively. For NdO⁺ and UO⁺ the excitation energies for the Ω states were calculated with a valence configuration interaction method using ab initio effective spin—orbit operators to couple the molecular orbital configurations. The results for NdO⁺ are very comparable with the results for UO⁺, and show the vibrational and electronic states to be interleaved.