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" To: bravel@bnl.gov Dear Prof. Ravel, I'm working on data coming from XAFS measurements carried out at Pb-L3 edge on lead containing glassy samples. Three reducing agent are used to reduce lead(+II) inside the original glass at high temperature. Our samples contain then a fraction is in fcc lead(0) form and one other fraction of lead(+II) still remaining inside the glassy framework. EXAFS analysis show a decrease of the Pb-O coordination number from 3 (for original glass) to lower CN (1 in extreme case). My major concern is the analysis of XANES, some modifications are observed on the finger prints. I've try to fit the spectra using linear combination fitting module from Athena using lead and original glass spectrum with poor result as you have understand. I need then to generated model XANES spectra for the CN. As I'm not an expert of FEFF, I need some advices. Can I use a cluster PbOx ((d=2.23 A) with coordinate for Pb and O coming from lead oxide) containing few atoms? Is the obtained spectra reliable to give a "good" fit? I hope my questions are clear for you. Thanks in advance for your remarks. Best regards Dr. Pascal Yot ------------------------------------------------------- -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/