On Tuesday 27 March 2007 14:34, Mauro Rovezzi wrote:
Anatoly,
I don't quote your answer because I would like to add some more elements to specify what I'm searching for.
In my previous post I have used the term "cluster" in a wrong way, that is I was thinking at the crystal-box (normally less than 216 atoms) used in the ab-initio density functional theory calculations and "doped" with few percent (< 10%) of an absorbing atoms. For example, if we put 5 absorbing atoms in a box of 64 (that represent the crystal host) we have 5 possible configurations for our FEFF calculation and is impossible in an EXAFS fit to include all the important paths (too many!).
My idea to take into account the numerical simulation and combine it with the experimental EXAFS data could be to use the pair radial distribution function extracted from the simulation, average the configurations, recalculate paths and proceed in the fit.
Mauro, If I understand what you are trying to do, this paper by me might be relevant: Role of local disorder in the dielectric response of BaTaO2N, B. Ravel, Y-I. Kim, P.M. Woodward, and C.M. Fang, Physical Review B, 73, p. 184121 (2006) I used a DFT result as the basis of my fitting model in that paper. The naive approach was to make a Feff path from every absorber/ scatterer pair in the box. That's certainly a good idea, but Ifeffit has some compiled-in limitations that made that impractical. My solution was to make an evenly spaced grid in R and bin together paths with similar path lengths. That reduced the problem to a tractable number of Feff paths at the expense of writing a one-off program as a processing step between the DFT and setting up the EXAFS fit. If my answer is relevant to your problem, I have a few other ideas about how to approach the problem that we can discuss futher, if you'd like. B -- Bruce Ravel ---------------------------------------------- bravel@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/