next up previous contents
Next: FF2CHI: XAFS spectrum Up: Input File Controls Previous: PATHS: Path enumeration

GENFMT: XAFS parameters


For each path the code calculates the effective scattering amplitude (tex2html_wrap_inline3327 from which FEFF gets its name, see Section 3.2.4) and the total scattering phase shift along with other XAFS parameters using the scattering matrix algorithm of Rehr and Albers. Once the scattering phase shifts and the paths are determined, no other input is necessary for this calculation.

 CRITERIA    critcw critpw    (Useful)

Since the number of multiple scattering paths gets large very quickly, it is necessary to eliminate as many paths as possible. Fortunately, we have found that most multiple scattering paths have small amplitudes and can be neglected. Various cutoff criteria are used in FEFF8 to limit the number of paths to consider. These criteria are based on the importance of the path, defined as the integral over the full energy range of tex2html_wrap_inline3329. Very close to the edge these cutoff criteria should be examined with care and in some cases reduced from the values used for EXAFS.

critcw is the cutoff for a full curved wave calculation. A typical curved wave calculation requires a complete spherical wave calculation, which typically takes seconds of CPU time per path. The default value of critcw is 4%, meaning that any path with mean amplitude exceeding 4% of largest path will be used in calculation of chi. The criterion critcw is used by GENFMT. Since the XAFS parameter calculation is already done, the savings is not in computer time, but in disk space and ease of analysis. The values of critcw for each path are written in the file 'list.dat' written by module GENFMT.

critpw is a plane-wave approximation to tex2html_wrap_inline3331. This is extremely fast to calculate, and is used in the pathfinder. The default value of critpw is 2.5, meaning that any path with mean amplitude 2.5% of largest path, including degeneracy factors, (in plane wave approximation) will be kept. Any path that does not meet this criterion will not be written to 'path00.dat', and there is no need to calculate the XAFS parameters for this path. The default for critpw is less than that for critcw since some paths are more important when the full curved wave calculation is done than they appear in the plane wave approximation. Since the plane wave estimate is extremely fast, use this to filter out as many paths as you can. The file 'crit.dat' (written by the module PATHS) tells you critpw for each path that passes the criterion.

The method of calculation of these importance factors has been improved for FEFF8, so don't worry if the values for some paths have changed slightly from previous versions. (Default values critcw=4.% critpw=2.5%)

CRITERIA  6.0  3.0   * critcw 6%, critpw 3%
CRITERIA  0    0     * use all paths (cw and pw criteria turned off)

 NLEG    nleg    (Useful)

The NLEG card limits the number of scattering paths to nleg. If nleg is set to 2, only single scattering paths are found. The default is nleg = 8.

* only single scattering paths (i.e. 2 legged paths)

 IORDER    iord    (Advanced)

Order of the approximation used in module GENFMT. FEFF uses order 2 by default which is correct to terms of order tex2html_wrap_inline3333, and corresponds to 6x6 scattering matrices in the Rehr-Albers formalism. Single scattering is calculated exactly to this order. The 6x6 approximation is accurate to within a few percent in every case we have tried (that is, higher order doesn't change the result more than a few percent). However tex2html_wrap_inline3335 shells and higher shells may require increased iorder for coupling the matrix elements. Changing the default values requires some familiarity with the Rehr-Albers paper and the structure of the module GENFMT. To do so, follow the instructions in the FEFF source code in subroutine setlam. The key iord is passed to setlam for processing. You may need to change the code parameter lamtot if you want to do higher order calculations. For details of the algorithm used by GENFMT, see the paper by J.J. Rehr and R.C. Albers (see the references in Appendix C). For the tex2html_wrap_inline3335 and higher edges, you may receive the error message like: Lambda array overfilled. In that case the calculations should be repeated with IORDER -70202 (10x10 matrices).
* change iorder for M4 calculations
IORDER -70202

 NSTAR        (Advanced)

When this card is present, GENFMT will write the file 'nstar.dat' with the effective coordination number tex2html_wrap_inline3339 which is the coordination number weighted by tex2html_wrap_inline3341 to correct for polarization dependence in SEXAFS calculations.

next up previous contents
Next: FF2CHI: XAFS spectrum Up: Input File Controls Previous: PATHS: Path enumeration
John J. Rehr
Thu Jul 1 14:31:17 PDT 1999