On Thursday 29 March 2007 16:58, Ricardo Faccio wrote: > I am performing a refinement for a La2CuO4 in powder. > I followed Bruce's course, and I tried determine S02 hopping its > correlation with sigma^2. > I am attaching a graph of this determination. Please see enclosed the file. > My question is: Is common to obtain a S02 of 0.625? and is common to obtain > a nearly sigma^2=0?
That doesn't seem right. So2 and sigma^2 are correlated such that a low value of one can be compensated by a low value of the other. I'd guess that is happening here. > I didn't fit E0, alpha and degeneracy for the first single shell. What is > the best strategy to fix this bad result? It is worth mentioning that the document you are referencing is rather old. The issue of the technique you are asking about has come up several times on this mailing list with various people casting doubt on how reliable or believable it is. I certainly prefer to use other tactics these days. Much more reliable, to my mind, is the multiple k-weights approach to the fit where you set up the fit with all the parameters you think you need then tell Artemis/Ifeffit to use many values of the k-weight. See page 8 of http://cars9.uchicago.edu/~ravel/talks/artemis.methyltin.pdf. I think that is a much more reliable approach to figuring out the correlations in So2 and sigma^2 in that the rest of the fit happens at the same time. As you suggest, not fitting the other parameters can have an impact on your results. B -- Bruce Ravel ---------------------------------------------- [EMAIL PROTECTED] 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/ _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
