Pascal,
I am answering your question on the Ifeffit mailing list, which is the
most appropriate venue for questions of this sort. Please read this
page:
http://cars9.uchicago.edu/iffwiki/BruceRavel/FormLetters/HelpRequest
If I understand your question, you are asking about whether it is
appropriate to run Feff with a small cluster. The canonical answer to
that question is that you want to use a large cluster whenever
possible so that the muffin tin potential is well constrained.
Sometimes, with a very small cluster, the algorithm Feff uses to
compute the muffin tin radii can result in radii that are unphysically
large, resulting in an unphysical calculation.
Of course, that is a somewhat hand-waving answer. It is hard to know
without actually running a calculation whether the problem with muffin
tin radii will actually happen. I have comfortably published results
which use very small clusters indeed.
In your case, as in so many cases, the best advice I can give is to
try it both ways and see what happens. That is, you can try doing a
XANES calculation (Are you using Feff8? You didn't say...) with the
small cluster and comparing it to the Feff8 calculation which uses a
large cluster to compute the potentials and small cluster to compute
the full multiple scattering XANES. See the Feff document at
http://leonardo.phys.washington.edu/feff/wiki/index.php?title=FEFF_Documenta...
for details about how to set parameters in the feff.inp file to make
that happen.
Regards,
B
---------- Forwarded Message ----------
Subject: Questions about Athena & FEFF
Date: Wednesday 15 April 2009
From: "Pascal G. YOT"