ABINITIO CURVED-WAVE X-RAY-ABSORPTION FINE-STRUCTURE

The most important elements of ab initio calculations of x-ray-absorption fine structure (XAFS) are studied. To obtain accurate results without ad hoc adjustable parameters, we find it essential to include (i) curved-wave effects, (ii) a complex, energy-dependent self-energy, (iii) an approximate molecular potential, and (iv) a fixed energy reference for the photoelectron wave number. Based on these findings, an automated code has been developed for ab initio calculations of single-scattering XAFS, in which curved-wave effects are treated exactly in terms of effective backscattering amplitudes, inelastic losses and self-energy shifts are incorporated with use of a Hedin-Lundqvist self-energy, an automated relativistic overlapping-atom muffin-tin potential is used, and the energy threshold is estimated from electron-gas theory. The efficiency of the code is made possible by analytic expressions for the Hedin-Lundqvist self-energy. This code replaces existing tables of XAFS phases and scattering amplitudes and yields reliable theoretical XAFS standards for arbitrary pairs of atoms throughout the Periodic Table (Z less-than-or-equal-to 94). These results are comparable to those from self-consistent calculations and are valid to within about 20 eV of the absorption edge. Comparisons with experiment are presented for Cu, Ge, Pt, Br2, and GeCl4. The calculated XAFS amplitudes are found to be accurate to within 15%; XAFS phases are accurate to within 0.2 rad; and nearest-neighbor distances are typically accurate to within 0.02 angstrom.