Efficient core-excited state orbital perspective on calculating X-ray absorption transitions in determinant framework

TL;DR

This paper introduces a reformulated MBXAS approach for X-ray absorption spectroscopy that simplifies calculations by using core-excited state orbitals, improving convergence, reducing computational costs, and enabling better interpretation of spectral features.

Contribution

The authors reformulate the MBXAS method using core-excited state orbitals, offering practical advantages and insights into the spectral intensity contributions in XAS.

Findings

Reformulated MBXAS avoids convergence issues with unoccupied orbitals.

The new approach reduces computational expense by eliminating the need for unoccupied orbitals.

Auxiliary orbitals help explain spectral intensity and compare many-body and single-particle treatments.

Abstract

X-ray absorption spectroscopy (XAS) is an explicit probe of the unoccupied electronic structure of materials and an invaluable tool for fingerprinting various electronic properties and phenomena. Computational methods capable of simulating and analysing such spectra are therefore in high demand for complementing the experimental results and for extracting valuable insights therefrom. In particular, a recently proposed first-principles approach titled Many-Body XAS (MBXAS), which approximates the final (initial) state as a Slater determinant constructed from Kohn-Sham (KS) orbitals optimized in absence (presence) of the relevant core-hole has shown promising prospects in evaluating the transition amplitudes. In this article, we show that the MBXAS approach can be rederived using a transition operator expressed entirely in the basis of core-excited state KS orbitals and that this reformulation offers substantial practical and conceptual advantages. In addition to circumventing previous issues of convergence with respect to the number of unoccupied ground-state orbitals, the aforementioned representation reduces the computational expense by rendering the calculation of such orbitals unnecessary altogether. The reformulated approach also provides a direct pathway for comparing the many-body approximation with the so-called single-particle treatment and indicates the relative importance in observed XAS intensity of the relaxation of the valence occupied subspace induced by the core excitation. Finally, using the core-excited state basis, we define auxiliary orbitals for x-ray absorption and demonstrate their utility in explaining the spectral intensity by contrasting them with single-particle approximations to the excited state.