For each path the code calculates the effective scattering amplitude ( 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 . 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 . 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)

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) NLEG 2

Order of the approximation used in module GENFMT. FEFF uses order 2 by default which is correct to terms of order , 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 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

* change iorder for M4 calculations IORDER -70202

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

Thu Jul 1 14:31:17 PDT 1999