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question

Stéphanie Belin asked:

 SB> Hello, I would like to ask you a question about ATOMS and FEFF.
 SB> I want to calculate an exafs spectrum of a non-stoichiometric
 SB> coumpound, and my problem is that ATOMS doesn't take into account
 SB> occupation ratio (fractional occupancies) on the cristallographic
 SB> positions of atoms.  So, I would like to know the best way to
 SB> proceed ?  Moreover, these positions are occupied by two
 SB> different atoms ...  I tried to modify FEFF.INP file, but It
 SB> can't reproduce the real structure.  Thank you for your
 SB> attention.  Sincerely yours.


The explanation of why Atoms does not and *cannot* translate
occupation fractions into the feff.inp file can be found in the Atoms
document.  Here is the URL that jumps straight to the explanation:

  http://leonardo.phys.washington.edu/~ravel/software/atoms/doc/Atoms/node4.html#SECTION00442000000000000000

In my course on EXAFS analysis, I discuss the problem of mixed
shells.  The course home page is

  http://leonardo.phys.washington.edu/~ravel/course/

I describe some techniques for solving this problem using feff and
feffit in Chapter 10 of this PDF file:

  http://leonardo.phys.washington.edu/~ravel/course/



The short answer is that it is somewhere between hard and impossible
to properly model exafs on a mixed shell with a single feff run.  My
solution described above requires running feff two or more times and
combining the output of those feff runs using your fitting program.

Check out that stuff and if that doesn't help, post here again and
hopefully I or someone else will offer some useful wisdom for you.

Good luck
Bruce


-- 
 Bruce Ravel  ----------------------------------- ravel@phys.washington.edu
 U.S. Naval Research Laboratory, Code 6134          phone: (1) 202 767 5947
 Washington DC 20375, USA                             fax: (1) 202 767 1697

 NRL Synchrotron Radiation Consortium (NRL-SRC)
 Beamlines X11a, X11b, X23b, X24c, U4b
 National Synchrotron Light Source
 Brookhaven National Laboratory, Upton, NY 11973

 My homepage:    http://feff.phys.washington.edu/~ravel 
 EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/