A. L. Ankudinov, MST-11, Los Alamos National Laboratory.

Accurate potential construction is a crucial step for quantitative calculations of various x-ray spectroscopies (EXAFS, XANES, XES, XPS, etc.). For EXAFS spectroscopy it was found that simple superposition of atomic densities leads to accurate calculations. However, situation is different closer to the edge, where this simple prescription [often]ceases to work well. Present state-of-the-art electronic structure codes usually employ self-consistent full potential calculations. Thus potential and densities can be obtained from those codes (e.g. FP LAPW code) for crystalline materials. Since many materials of current interest (CMR, HTSC) are disordered, [the] RSMS approach, traditionally used for EXAFS, should be modified to include full potential as well as self-consistency.

Essential in x-ray spectroscopy calculations are core-hole and self-energy effects. Thus within the final state rule the x-ray absorption should be calculated with core-hole and x-ray emission without. Typically this rule works well, but the dynamics of core-hole screening is not yet well investigated. The Self-energy is a complex energy dependent exchange-correlation potential. Currently used Dirac-Hara and Hedin-Lundquist models are based on the uniform electron gas calculations. A better way is provided by the GW approximation, which is also currently used to (search) provide an improvement over the LDA approximation for ground state exchange-correlation potential. Within this approximation exhange- correlation potential is material specific, however it requires significantly more computational time.

These improvements of the potential construction are necessary for quantitative analysis of XANES and other near edge spectroscopies.