Computation of M-Edge X-ray absorption spectra using relativistic multireference methods

X-ray absorption near edge structure (XANES) spectroscopy has become a powerful tool to provide detailed information of the local electronic structure and oxidation state of a system. The metal M_4,5 edge XANES is a direct probe of f orbitals, and as such can also provide insight into the role of f orbitals in metal-ligand bonding in lanthanides and actinides. The theoretical prediction and interpretation of metal M-edge X-ray absorption spectra is complicated by both the highly multiconfigurational nature of the states involved and by significant relativistic effects, including spin-orbit coupling. In this work we present calculations of the M_4,5-edge of lanthanide complexes, using a recently developed two-component multireference configuration interaction scheme, where relativistic effects are included variationally. Results demonstrate the importance of the multireference nature and multi-electron processes in the prediction of the spectrum.