Re: [Ifeffit] Ifeffit Digest, Vol 49, Issue 20
Dear Iffefit users 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? I didn't fit E0, alpha and degeneracy for the first single shell. What is the best strategy to fix this bad result? Thanks in advance Ricardo ------------------------------------------------------------------------- ----- Ing. Quím. Ricardo Faccio Mail: Cryssmat-Lab., Cátedra de Física, DETEMA Facultad de Química, Universidad de la República Av. Gral. Flores 2124, C.C. 1157 C.P. 11800, Montevideo, Uruguay. E-mail: rfaccio@fq.edu.uy Phone: 598 2 9241860 Int. 109 598 2 9290705 Fax: 598 2 9241906 Web: http://cryssmat.fq.edu.uy/ricardo/ricardo.htm
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 ---------------------------------------------- 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/
Hi Ricardo, I'll add one thing to Bruce's comment: Do you care what the sigma^2 is? I mean, other than to hope it's "good," is it part of the information you want to know about your sample? If it's not, and if the uncertainties and correlations reported by Artemis suggest that the best fit is simply finding a value of S02 and sigma2 that are both too low, and if the parameters you are interested in do no correlate strongly to S02 or sigma2, then it's not a particularly serious problem. You could just set S02 to the value found for a standard, or something like that, and as long as the fit is still decent in other respects, not worry about it further. I realize I strung together 3 "if's" there, but it's still a pretty common set of circumstances. In your case, the first "if" looks OK from your graph--an S02 of 0.8 or so shows error bars from the three k-weights that nearly overlap and correspond to a reasonable value of sigma2. --Scott Calvin Sarah Lawrence College At 06:18 PM 3/29/2007, you wrote:
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.
participants (3)
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Bruce Ravel -
Ricardo Faccio -
Scott Calvin