Many-body corrections in self-consistent field approaches to X-ray spectroscopy simulations

X-ray spectroscopy provides unrivaled element-specifc chemical information on materials and their interfaces, with relevance to device physics, surface chemistry, and materials functionality. Interpretation of such measurements can be challenging due to a lack of spectral fingerprints, particularly in the case of operando measurements. Accurate excited-state electronic structure methods have been key to unravelling the underlying physics and chemistry present in measured spectra and, concomitantly, these experiments test our ability to derive representative models that are accurate and computationally efficient, within existing theoretical frameworks. For example, density functional theory can be used to model core-excited states self-consistently with constrained orbital occupancies (so-called core-hole approaches) implying a single-determinant approximation for excited states. We have explored this implication explicitly to reveal the importance of many-electron contributions to X-ray transition amplitudes, beyond single-particle approximations [1,2]. Initial results revealed the importance of such many-body contributions in adjusting the intensities of X-ray absorption features at the absorption onset and showed that in certain cases single-particle transitions alone cannot reproduce experimental findings. More recently, the many-body approach has been extended to X-ray emission [3] and Resonant Inelastic X-ray Scattering (when coupled with many-body perturbation theory) [4]. Our most recent findings indicate significant benefits in terms of accuracy and physical insight when using the core-excited orbital representation within the many-body framework [5].