Hybrid QM/QM Study of Electronic d-d Spectra of Trivalent Transition Metal Oxides

Transition metal oxides have been traditionally studied in theoretical chemistry and solid-state physics as they reflect the complexity of the electronic structure of materials with open $d$ -shells. The complex electronic structure stems from the presence of strong correlations of both static and dynamical nature which are characteristic of open shell $d$ -electrons. Systematic and computationally efficient account for correlation effects in transition metal compounds is a key challenge in theoretical chemistry. Hybrid electronic structure methods represent a promising way to address this problem. In this work, we apply a hybrid QM/QM method-periodic Effective Hamiltonian of Crystal Field (pEHCF)-to study multiplet structure of open $d$ -shells in a series of trivalent and mixed-valence oxides: $\alpha$ -Fe₂O₃, $\alpha$ -Mn₂O₃, Cr₂O₃ and Co₃O₄. The calculated energies of the excited $d$ -multiplets are compared with available experimental optical spectroscopy data. Our calculations clarify the interpretation of spectroscopic lines observed in experiments for two challenging systems: Fe₂O₃ and Co₃O₄. Furthermore, we assess the overall performance of pEHCF for transition metal oxides and place our results in a more general context by comparing them with previous studies that used different electronic structure methods.