Accurate relative core-level binding energies from pseudopotential DFT with a modified static GW approximation

The binding energy of an electron in a given core-level can vary by several eVs, depending on the chemical environment around the atom. These chemical shifts contribute to the peak positions in experimentally measured x-ray photoemission, absorption, and emission spectra. Observed shifts in experimental spectra are used to identify atomic-scale materials properties. First-principles calculations are used to predict core-level shifts using a variety of techniques, including quantum chemistry approaches, many-body perturbation theory or GW, and delta self-consistent field or constrained occupancy density-functional theory (DFT). Each of these approaches has different trade-offs in terms of accuracy and accessible system size. Here we present an efficient GW approximation for determining relative core-level shifts on top of pseudopotential DFT calculations, using a modification of the OCEAN spectroscopy code. We benchmark our results against a set of small molecules, but this approach is easily applied to surfaces and bulk systems.