Optical excitations in hematite (α-Fe₂O₃) via embedded cluster models: A CASPT2 study

Hematite is a candidate for use as a photoanode in water-splitting reactions. Part of the efficiency depends on the capability of hematite to absorb sunlight and convert solar energy to electron energy. While there have been optical spectroscopy measurements as well as molecular orbital studies on the optical spectra of hematite, accurate characterizations of excited states from theory are missing. To fill this gap, we study excited states of electrostatically embedded hematite clusters using complete active space self-consistent field theory and complete active space with second-order perturbation theory. Overall, we found that the lowest lying excitations within hematite are Fe d-d ligand field transitions (starting at ∼2.5 eV), which are highly localized around Fe centers. The O 2p to Fe 3d ligand to metal charge transfer excitations are higher lying excited states (∼6 eV). These excitation energies are used to verify some of the earlier peak assignments for the optical spectra of hematite. In addition, we demonstrate that density functional theory energy differences between different spin states of Fe in the embedded FeO₆^9- cluster are significantly biased by the choice of exchange-correlation functional. Therefore, any conclusions derived from those types of calculations should be viewed with caution.