feff

FEFF is a suite of computer programs used in X-ray absorption spectroscopy to calculate XANES and EXAFS spectra for a chosen atomic cluster around an absorbing atom. It implements real-space multiple-scattering theory to model the scattering of the photoelectron by neighboring atoms.

The calculations can include various physical effects such as core-hole treatment (final-state effects) and different potential approximations (muffin-tin or full-potential variants in later versions). FEFF can compute spectra for different absorption edges (K, L) and outputs energy-domain mu(E) or k-space chi(k) and R-space chi(R) representations.

Typical workflow: provide a structural model (positions and species of atoms in the cluster), select the absorber, set calculation parameters, run FEFF to generate theoretical spectra, and compare with experimental data. FEFF results are commonly analyzed with external fitting tools such as IFEFFIT to extract local structural information from experimental XAFS data.

FEFF has been widely used since the 1990s and has evolved through multiple versions. It remains a standard reference in XAFS analysis and is distributed with accompanying utilities for data handling and visualization.