Calculation Strategies and Examples

Self-consistent or overlapped atom potentials are necessary for the calculation of the scattering phase shifts. Self-consistent calculations take more time, and are often essential for XANES, especially for cases with significant charge transfer. Although the effect of self-consistency on EXAFS is small, such calculations give an accurate determination of E0, thus eliminating an important parameter in EXAFS distance determinations.

Scattering phase shifts for each unique potential are necessary for FMS, PATHS and GENFMT. They are needed for the importance filters in PATHS and are the basis of the XAFS parameters calculation in GENFMT. This part of the calculation is relatively slow, so it is usually best to run it only once and use the results while studying the paths and XAFS.

To enumerate the necessary paths, the pathfinder module PATHS needs the atomic positions of any atoms from which scattering is expected. If the structure is completely unknown, only single-scattering paths can be created explicitly. Because the number of possible paths increases exponentially with total path length, one should start with a short total path length, examine the few paths (representing scattering from the nearest neighbors), and gradually increase the total path length, possibly studying the path importance coefficients and using the filters to limit the number of paths. This process is not automated, and if done carelessly can yield so many paths that no analysis will be possible.

Finally, use GENFMT to calculate the XAFS parameters, and FF2CHI to assemble the results into a chi curve. Here, the slow part is GENFMT and FF2CHI is very fast. Therefore, to explore parameters such as Debye–Waller factors, mean free path and energy zero shifts, various combinations of paths and coordination numbers, run only module FF2CHI using the results saved from GENFMT.

There are three ways to modify the Debye–Waller factor, all of which affect only the module FF2CHI. The DEBYE card calculates a Debye–Waller factor for each path. The SIG2 card adds a constant Debye–Waller factor to each path. And you can edit ‘list.dat’ to add a particular Debye–Waller factor to a particular path. These three Debye–Waller factors are summed, so if the DEBYE and SIG2 cards are present, and if you have added a Debye–Waller factor to a particular path, the Debye- Waller factor used will be the sum of all three. See documentation below for details.

If your model changes significantly, the phase shifts (which are based in part on the structure of the material) should be recalculated. Any time the phase shifts change, the XAFS parameters will also have to be re-calculated. If the path filters have been used, the path list will also have to be recomputed.

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