Time-dependeint density functional theory calculations of X-ray absorption

There has been dramatic progress in recent years both in calculations and in the interpretation of X-ray absorption spectra (XAS). Often an independent-electron approximation with final state potentials is adequate. However, for soft X-rays (i.e., energies less than about 1 keV) local field effects can be important. Such local fields arise from the dynamic screening of both the external X-ray field and the coupling to the core hole created in the absorption process. These effects require a theory that goes beyond the independent-electron approximation. We developed an efficient approach for treating such effects in molecules and solids based on a generalization of time-dependent density functional theory (TDDFT), with a local approximation for the screening response. The approach has been implemented in our self-consistent, real-space Green's function code FEFF8 in terms of screened dipole transition matrix elements. Typical results are discussed for the XAS of the N-4,N-5 edges of solid Xe and for the L-2,L-3 edges of 3d transition metals. Our approach accounts for the deviations of the L-3/L-2 intensity branching ratio from the 2:1 value of the independent electron approximation. For the N-4,N-5 edges of Xe, the approach also accounts for the observed fine structure. (C) 2003 Wiley Periodicals, Inc.