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/