Non-local self-energy models for XAS

Ab initio calculations of X-ray absorption spectra (XAS) and its fine structure (XAFS) are dependent on the many-body exchange and correlation interaction between the photoelectron and the Fermi sea. Non-local Hartree-Fode (HF) exchange provides a better description of exchange with core electrons while the local density approximation (LDA) is usually better for valence electrons. The GW approximation for the self-energy Sigma provides a systematic way of joining HF and LDA, using the natural separation of Sigma into core and valence parts. This exchange model is implemented in FEFF7, a new Dirac-Fock based generalization of the FEFF6 code for XAS calculations. Improvements are found compared with LDA based codes, namely more accurate excitation energies, phase shifts, amplitudes. These lead to improved near edge calculations and improved distance and coordination number determinations from XAFS analysis. Spin dependent self-energies and applications to XMCD are also discussed.