Restricted Open-Shell Configuration Interaction Singles Study on M- and L-edge X-ray Absorption Spectroscopy of Solid Chemical Systems

In this study the M- and L-edge X-ray absorption spectra of a series of open- and closed-shell solids (TiO2 rutile, α-Fe2O3 hematite, FeS2 pyrite, and the spinel Co3O4) are investigated with the restricted open-shell configuration interaction singles methods (ROCIS/DFT and PNO-ROCIS/DFT) using the embedded cluster approach. ROCIS/DFT type of methods are grounded in wave function-based ab initio electronic structure theory and have shown great performance in the field of X-ray spectroscopy in particular in the field of transition metal L-edge spectroscopy. In this work we show that ROCIS/DFT can be used to calculate and interpret metal M- and L-edge XAS spectra of solids. To this end, clusters with up to 52 metal centers are considered. In all cases good to excellent agreement between theory and experiment is obtained. The experimentally probed local coordination environments are discussed in detail. The physical origin of the observed spectral features is explored through the machinery of natural difference orbitals. This analysis provides valuable information with respect to the core to valence, metal to metal charge transfer, and metal to ligand charge transfer characters of the relativistically corrected many particle states. The influence of the above electronic effects to the spectral shapes and the size of the treated clusters are thoroughly investigated.