Category:Scattering potentials

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ATOMIC, POT, and SCREEN: Scattering potentials

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. feff9 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. 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.

Note that feff has historically accumulated a bunch of options for setting the edge and core hole. We highly recommend that users use only the EDGE card for specifying the edge, and the COREHOLE card for setting the core hole treatment. The HOLE and NOHOLE cards are then provided for backward compatibility only.

Control Card Categories

  1. Structural Information Cards
  2. Spectrum Information Cards
  3. General Cards
  4. POT: Scattering potentials
  5. XSPH: Cross-section and phase shifts
  6. FMS: Full multiple scattering
  7. PATHS: Path enumeration
  8. GENFMT: XAFS parameters
  9. FF2CHI: XAFS spectrum
  10. SFCONV: Spectral Function Convolution
  11. EELS: EELS spectrum

Articles in category "Scattering potentials"

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