Initially the free atom potentials of each atomic type are calculated using a relativistic Dirac-Fock atom code, treating the atoms as if they were isolated in space. Scattering potentials are calculated by overlapping the free atom densities in the muffin tin approximation (Mattheiss prescription), and then including the Hedin-Lundqvist/Quinn self energy for excited states. Non-overlapping muffin-tin radii are determined automatically from the calculated Norman radii. Automatic overlapping of muffin tin spheres (see the AFOLP card) is done by default, since it typically leads to better results than non overlapping muffin-tin spheres. FEFF8 can also calculate self-consistent potentials by successively calculating the electron density of states, electron density and Fermi level at each stage within a small cluster and then iterating, using the Mattheiss prescription for the initial iteration.
XAFS spectra are referenced to the threshold Fermi level. This quantity is best determined with the self-consistent field procedure (typically to within a fraction of an eV), or (less accurately but faster) can be estimated from the electron gas result at the mean interstitial density in the Mattheiss prescription, as in FEFF7. An absolute energy scale is obtained by an atomic calculation of the total energy of the system with and without the core-hole. Atomic configurations and core-hole lifetimes are built in, and mean free paths are determined from the imaginary part of the average interstitial potential, including self-energy and lifetime contributions.
The potential calculations need as input only the atomic number of the atoms, and, for the absorbing atom, the type of the core hole being considered. To do the overlapping of the unique potentials, the neighboring atoms must be identified, either by position (from a list of the Cartesian coordinates of each atom) or by explicit overlapping instructions using the OVERLAP card described in the previous section.
To save time the code calculates the overlapped atom potential for each unique potential only once, using as a sample geometry the atom with a given unique potential index that is closest to the absorbing atom. Thus it is essential that the neighborhood of each sample atom be appropriate.