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.
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%)
This 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 the largest path will be used in
the calculation of . The criterion critcw is used by
Since the XAFS parameter calculation is already done, the savings is
not in computation 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.
This 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 exceeding 2.5% of the 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 `paths.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 path) tells you
critpw for each path that passes the criterion.
CRITERIA 6.0 3.0 * critcw 6%, critpw 3%
CRITERIA 0 0 * use all paths (cw and pw criteria turned off)