Relativistic Orbital Optimized Density Functional Theory for Accurate Core-Level Spectroscopy

TL;DR

This paper introduces a relativistic orbital optimized DFT method combined with the X2C model, achieving high accuracy in predicting core-level spectra of heavier elements with significantly reduced errors compared to traditional TDDFT.

Contribution

The study develops and validates a relativistic OO-DFT/X2C approach that accurately predicts core-level spectra of third period elements without empirical shifts, outperforming existing methods.

Findings

Achieves ~0.5 eV RMS error in spectra prediction

Significantly improves over >50 eV deviations of TDDFT

Successfully reproduces experimental spectra without empirical shifts

Abstract

Core-level spectra of 1s electrons of elements heavier than Ne show significant relativistic effects. We combine advances in orbital optimized DFT (OO-DFT) with the spin-free exact two-component (X2C) model for 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 $\sim 0.5$ eV RMS error vs experiment with the modern SCAN (and related) functionals. This marks a significant improvement over the $>50$ eV deviations that are typical for the popular time-dependent DFT (TDDFT) approach. Consequently, experimental spectra are 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 promising route for computing core-level spectra of third period elements. We also explored K and L edges of 3d transition metals to identify limitations of the OO-DFT/X2C approach in modeling the spectra of heavier atoms.