On Wednesday 01 February 2006 13:14, Renato Canha Ambrosio wrote:
As described in page 80 of the EXAFS ANALYSIS WITH FEFF AND FEFFIT - PART 2: COMMENTARY by Bruce Ravel the eins function is a function evaluated in the context of the current path. So the reduced mass is calculated with the mass of the atoms in the path. Its ok but I have several scans at different temperatures so I would like to refine not only the Theta (Einstein temperature) but also the reduced mass and perhaps the static disorder. Can I do it in the context of the path or I have to perform a non - xafs refinement with the file containing the temperatures and sigma^2. I make the same question for the Debye correlated model.
That's a very interesting question. I have a few suggestions of things to think about. 1. Considering static disorder in the context of a fit with ifeffit and the einstein model is pretty straight forward. You can use a math expression like eins(temp, thetaE) + sig_static then use thetaE and sig_static as quess parameters. Those two terms might prove to be rather highly correlated, but if you do a multiple data set fit with enough temperatures, you'd have a good chance of breaking the correlations. 2. You could just float a single parameter that is used for all parts of the sigma^2, then analyze that parameter as a function of temperature once the fits at all temperatures are complete. I think that is what you mean by a "non-xafs refinement". That has the advantage of being flexible and unburdened by the requirements of doing the XAFS fit. 3. You could do the einstein model but not use Ifeffit's eins function. The formula for the eins function is given in Sevillano, Meuth, Rehr, Physical Review B 20:12 (1979) pp. 4908-4911. You could make math expressions in Ifeffit that encode that formula then use any part of it as a guess parameter. This might be a bit complicated and error-prone to implement, but is certainly possible. It has the advantage of being very general and you get to evaluate correlations between the terms you are actually interested in and the other terms in the fit. Hope that helps, 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/exafs/