Electronic structure calculations and X-ray spectroscopy simulations at European XFEL

In support of experiments at EuXFEL, we employ electronic structure theory in various forms: Density functional theory (DFT) is the work horse of electronic structure theory. It can deliver sufficiently accurate results for simple properties of uncorrelated materials and can be used as a basis for more advanced calculations. DFT-MD uses DFT to calculate interatomic forces feeding into a molecular dynamics simulation, which calculates lattice dynamics in Born-Oppenheimer approximation. Quasiparticle GW (QSGW) treats correlations perturbatively in a first order approach and recalculates the electronic structure including these corrections self-consistently. This is often sufficient to treat moderately correlated materials. Multiplet ligand-field theory using Wannier Orbitals: This methods focuses on the highly correlated orbitals and treats them with a model Hamiltonian, which is parameterized using Wannier orbitals calculated from DFT results. This approach includes correlations non-perturbatively, but it is very limited in the number of orbitals that can be treated without further approximations.