Ab Initio Calculations of Optical Constants
from UV to X-rays*
There has been substantial progress in recent years both in calculations and in the interpretation of x-ray absorption spectra (XAS). For deep core x-ray absorption an independent-electron approximation with final state potentials including a core-hole is often adequate . However, corrections to the independent particle approximation due to local fields are often important, even far above an absorption edge. These local fields arise
from the screening of the external x-ray field and the coupling to the core-hole. Thus their treatment requires theories that goes beyond the independent-electron approximation. We have developed an efficient approach for treating these effects in solids and molecules based on a combination of the Bethe-Salpeter equation (BSE) and time-dependent density functional theory (TDDFT) . This combined approach has been implemented within the
real space Green's function method, and makes possible efficient calculations of optical constants from the UV to x-ray energies. These include the dielectric function, XAS, and energy loss, as well as related quantities like x-ray scattering factors. The extension to finite momentum transfer (NRIXS) is also discussed .
 J. J. Rehr and R. C. Albers, Rev. Mod. Phys. 72, 621 (2000).
 A. L. Ankudinov, J. J. Rehr and Y. Takimoto, Phys. Rev. B 71, 165110 (2005).
 J. A. Soininen, A. L. Ankudinov, and J. J. Rehr, Phys. Rev. B (in press).
*Supported by DOE Grant DE-FG03-97ER45623 and facilitated by the DOE CMSN.