Systematic Investigation\nof Electronic States and\nBond Properties of LnO, LnO<sup>+</sup>, LnS, and LnS<sup>+</sup> (Ln\n= La–Lu) by Spin–Orbit Multiconfiguration Perturbation\nTheory

The electronic structures of lanthanide monoxides (LnO/LnO<sup>+</sup>) and monosulfides (LnS/LnS<sup>+</sup>) for all lanthanide\nseries elements (Ln = La–Lu) have been systematically analyzed\nwith sophisticated quantum chemical calculations. The ground electronic\nconfiguration has been determined to be Ln 4f<sup><i>n</i></sup>6s<sup>1</sup> or 4f<sup><i>n</i>+1</sup> for the\nneutral molecules and Ln 4f<sup><i>n</i></sup> for the cations.\nThe low-lying energy states resulting from spin–orbit coupling\nand ligand field effects have been resolved using spin–orbit\nmulticonfiguration quasi-degenerate second-order perturbation theory\ncalculations. The ionization energies of LnO (5.20–7.06 eV)\nare about 0.3–2.2 eV lower than those of LnS (5.54–9.22\neV) due to the difference in the Ln 6s and 4f orbital energies from\nwhich an electron is removed during the ionization process. The bond\ndissociation energies (BDEs) have been computed by the state-averaged\ngeneral multiconfigurational perturbation theory and the completely\nrenormalized coupled-cluster [CR-CC(2,3)] methods. The BDEs are highly\ndependent on the lanthanide elements as several factors of the lanthanides\naffect the bond dissociation. The calculated bond lengths and energies\nagree well with available experimental values and are systematically\npredicted for the series of lanthanide monoxides and monosulfides\nwhere experimental values are not available. Furthermore, the LS terms\nof low-lying energy states and their corresponding bond properties\nhave been clarified in detail to systematize the similarities and\ndifferences of the lanthanide compounds.