\textit{Ab initio} Quasiparticle Self-Energies and X-ray spectra

Present calculations of inelastic losses in x-ray spectra typically employ semi-empirical or highly simplified models, such as the plasmon-pole self-energy, which are only semi-quantitative for near edge spectra. Here we present an efficient {\it ab initio} approach applicable to general materials, starting from a real-space multiple-scattering calculation of the dielectric function,\footnote{M. P. Prange, J. J. Rehr, A. L. Ankudinov and J. A. Soininen, APS March Meeting 2006 (unpublished).} which is fit to a multiple-pole model with of order 10$^2$ poles. This yields multiple-pole GW self-energies, and hence related quantities such as inelastic mean free paths (IMFP). The approach leads to improved amplitudes and phases for core-level x-ray spectra up to photo-electron energies of order 10$^3$ eV. Results for the IMFP are found to be in good agreement with experiment and with other approaches.\footnote{E. L. Shirley, J. A. Soininen and J.J. Rehr, APS March Meeting 2006 (unpublished).}$^{,}$\footnote{C. J. Powell and A. Jablonsky, J. Phys. Chem. Ref. Data {\bf28}, 19 (1999).}