[Ifeffit] Using parameters in Artemis

Bruce Ravel bravel at anl.gov
Thu Mar 8 09:50:50 CST 2007

On Wednesday 07 March 2007 22:19, David Weedon wrote:
> The parameter "amp" is defined as the relative importance of the
> path and is assigned a value eg. 100 or 30, is this some form of
> percentage of this path to the first (major) path?  

That's correct.

When Feff computes the contributions from each path, it does a very
quick and dirty approximation of the amplitude of that contribution.
Typically, but not always, the amplitude of the first path is the
largest, so it gets an amplitude factor as reported by Feff of 100.
Each subsequent path gets the same quick 'n' dirty approximation of
size, which is reported as a percentage of the rough size of the
largest path.

The point of all that is to give you, the user, a sense of which paths
will contribute significantly to the EXAFS.  But it's just a guide to
the eye.  That amplitude approximation doesn't include sigma^2 or any
modifications to coordination numbers that you might find in your
actual data.  Sometimes paths that are reported as big paths turn out
to be very small contributions to the fit.  Also sometimes, the
small-ish paths turn out to be measurable in a fit.  It's just a guide
to the eye.


 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

 My homepage:    http://cars9.uchicago.edu/~ravel 
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

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