Relativistic orbital-optimized density functional theory for core-level spectroscopy

X-ray spectroscopy is widely used to study local chemical environments by probing the electronic structure of the innermost orbitals and obtaining element specific spectral signatures. Theory can assist in interpreting these features, but accurate modelling requires a description of orbital relaxation effects that are crucial for characterizing the core-hole. Moreover, as we explore elements of the periodic table with higher atomic numbers, relativistic effects become more relevant, especially for the prediction of K (1s) and L (2p) shell transitions. We combine orbital-optimized (OO) DFT with the spin-free exact two-component one electron model (SF-X2C1e) for scalar relativistic effects, to study K-edge X-Ray spectroscopies for elements of the third row and early transition metals of the periodic table. Our results show that the optimal protocol to simulate such effects is obtained when the SCAN density functional approximation is used with a local basis set that is flexible enough to describe the effects of orbital relaxation on the center of interest, predicting core-excitation energies to < 0.5 eV RMSE error for a wide range of molecules containing third row main group elements. We also present progress on the simulation of transition metal L-edges within this framework.