Relativistic Orbital-Optimized Density Functional Theory forAccurate Core-Level Spectroscopy

Core-level spectra of 1s electrons of elements heavier than Ne showsignificant relativistic effects. We combine advances in orbital-optimized densityfunctional theory (OO-DFT) with the spin-free exact two-component (X2C) modelfor scalar relativistic effects to study K-edge spectra of third period elements. OO-DFT/X2C is found to be quite accurate at predicting energies, yielding a similar to 0.5 eV root-mean-square error versus experiment with the modern SCAN (and related) functionals. Thismarks a significant improvement over the >50 eV deviations that are typical for thepopular time-dependent DFT (TDDFT) approach. Consequently, experimental spectraare quite well reproduced by OO-DFT/X2C, sans empirical shifts for alignment. OO-DFT/X2C combines high accuracy with ground state DFT cost and is thus a promisingroute for computing core-level spectra of third period elements. We also explored K and Ledges of 3d transition metals to identify limitations of the OO-DFT/X2C approach inmodeling the spectra of heavier atoms.