Accurate X-Ray Absorption Predictions for Transition Metal Oxides: An Advanced Self-Consistent-Field Approach Inspired by Many-Body Perturbation Theory

TL;DR

This paper compares $ abla$SCF and MBPT methods for X-ray absorption in transition metal oxides, finds limitations in $ abla$SCF, and proposes a many-body correction to improve spectral predictions, advancing first-principles spectroscopy.

Contribution

It introduces a many-electron approach to correct $ abla$SCF calculations, achieving accurate X-ray spectra beyond traditional methods.

Findings

$ abla$SCF underestimates pre-edge intensity in oxides.

MBPT predicts better spectral lineshapes.

Many-electron corrections improve $ abla$SCF results significantly.

Abstract

Constrained-occupancy self-consistent-field ($\Delta$SCF) methods and many-body perturbation theories (MBPT) are two strategies for obtaining electronic excitations from first-principles. Using the two distinct approaches, we study the O $1s$ core excitations that have become increasingly important for characterizing transition metal oxides and developing theory of strong correlations. Interestingly, we find that the $\Delta$SCF approach, in its current single-particle form, systematically underestimates the pre-edge intensity for chosen oxides, despite its success in weakly correlated systems. By contrast, the Bethe-Salpeter equation within MBPT predicts much better lineshapes. This inspires us to reexamine the many-electron dynamics of X-ray excitations. We find that the single-particle $\Delta$SCF approach can be rectified by explicitly calculating many-body transition amplitudes, producing X-ray spectra in excellent agreement with experiments. Our study paves the way to accurately predict X-ray near-edge spectral fingerprints for physics and materials science beyond the Bethe-Salpether equation.