Modeling X-ray Absorption Spectroscopy with Relativistic TDDFT

X-ray absorption spectroscopy (XAS) is an element specific probe that has been used to understand the local electronic and binding environment around metal centers. Electronic structure theory has become an important aid in interpreting XAS spectra. While K-edge spectra have been investigated in great detail, the modeling of L-edge spectra poses several unique challenges. First, the L-edge spectrum is composed of multiple features: L₁ corresponding to the 2s orbital, and L_2,3 corresponding to 2p orbitals, which are split into 2p_1/2 and 2p_3/2 levels by spin-orbit coupling. Additionally, with density functional theory (DFT) it also becomes necessary to make modifications to handle non-collinear spin densities in the presence of spin-orbit coupling. We model the L_2,3 spectra for several molecules with variety of model chemistries using exact two component TDDFT (X2C-TDDFT). With the X2C-TDDFT method we are able to include the one-electron spin-orbit coupling terms variationally from first principles. An analysis of the molecular orbitals involved in the transitions yields valuable theoretical information that can be used to connect the local electronic structure with experimental observables.