ARCHER2-eCSE04-3: Relativistic all-electron orbital-constrained Density Functional Theory to simulate x-ray photoemission and absorption spectroscopy

Core-hole calculations simulating x-ray photoelectron (XPS) or x-ray absorption spectra (XAS) are important to interpret complex experimental spectra. Density functional theory (DFT) is one method that can be used to efficiently and accurately simulate core-level spectra. The DFT code FHI-aims already has this functionality and has the added benefit of being able to produce absolute XPS binding energies, thanks to its all-electron numeric atomic-orbital basis approach. However, the code for these calculations was outdated and inefficient, not utilizing newer algorithms and software libraries. This project aims to bring the code up-to-date, improving the scaling and efficiency and restructuring and simplifying the code to make it more maintainable for future development. The new code achieves a significant improvement in the scaling of core-level simulations, on the same scale as standard single-point calculations, and improved the structure and readability of the code by removing almost 3000 lines of code and including new documentation and tutorials.