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From FEFF

FEFF is an automated program for ab initio multiple scattering calculations of X-ray Absorption Fine Structure (XAFS) and X-ray Absorption Near-Edge Structure (XANES) spectra for clusters of atoms. The code yields scattering amplitudes and phases used in many modern XAFS analysis codes, as well as various other properties.

FEFF 8: The newest version is FEFF8 (Version 8.40). FEFF 8 improves over previous versions of FEFF by including self consistent calculation of potentials to provide charge transfer and a more accurate estimate of the Fermi level. It also includes full multiple scattering calculations to improve XANES calculations, where multiple scattering expansions can fail to converge. FEFF 8 can output the angular momentum projected density of states, x-ray emission spectra and x-ray scattering amplitudes. The improvements in FEFF 8.40 simplify spin-dependent calculations and can include improved treatments of core-hole and local field effects.

FEFF 7: Relativistic version of FEFF 6 including polarization.

FEFF 6: High-order multiple-scattering EXAFS/XANES code.

ATOMS: Generates input files, feff.inp, for crystalline materials which are suitable for FEFF5 and higher (included with FEFF).

$\chi(k) = S^2_0 \mathcal{R}\sum_{shells}\frac{N F_{eff}}{k R^2}\exp(-2r/\lambda)\sin(2kR+\phi_{eff}+\phi_c)\exp(-2k^2\sigma^2)$