Hi everyone. I am currently trying to reduce the correlations between N and sigma squared. I have been told that multiple k-weight fitting is the way to go about this; but I am curious what the best method is to try and do this using Artemis. Is it simply a matter of clicking the other weighting options available? If so, what should I be looking for to determine whether my parameters are correct or not? I am not sure if this is a redundant question in terms of the mailing list; but I am a relatively new user and cannot remember seeing anything about it yet. Thanks. Dave Schmitter David C. Schmitter Dept. of Physics and Astronomy Nebraska Center for Materials and Nanoscience University of Nebraska Lincoln dschmitt@unlserve.unl.edu (402) 472-5094
On Monday 22 January 2007 11:50, David C. Schmitter wrote:
Hi everyone. I am currently trying to reduce the correlations between N and sigma squared. I have been told that multiple k-weight fitting is the way to go about this; but I am curious what the best method is to try and do this using Artemis. Is it simply a matter of clicking the other weighting options available? If so, what should I be looking for to determine whether my parameters are correct or not?
I am not sure if this is a redundant question in terms of the mailing list; but I am a relatively new user and cannot remember seeing anything about it yet.
Hi Dave, The intention of the mailing list is to have a forum for asking these sorts of questions. Multiple k-weighting is one of the standard methods in Ifeffit to attempt to introduce some information that uncorrelates N and sigma^2 (or deltaE and deltaR, for that matter). I recently gave a talk that touched upon this topic. Scroll to the bottom of this page: http://cars9.uchicago.edu/~ravel/talks/ and forgive the typos. (It was written on an airplane.) In short, multiple k-weights is very easy to introduce in artemis. Simply click more than one of the items in the the k-weight box in Artemis' data view (which is the one that shows up when the program is first launched). In that talk, I explain both how multiple k-weighting works in Ifeffit's fitting algorithm and give some hints as to why it works. You might also take a look at the tutorials by Matt and Shelly in the tutorials section of xafs.org. I think they both touch on this topic as well. As a final note, I should give the caveat that multiple k-weights and other tricks may help to reduce the correlations, but -- except in special situations -- propbably won't make them disappear. The amplitude parameters will always be correlated at some level, as will the phase parameters. It's just the nature of the problem. Even in a good, believable fit to a very simple problem like a metal foil, the correlations might be in the 80s. 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/
participants (2)
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Bruce Ravel
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David C. Schmitter