On Tuesday 12 July 2005 04:05, Stefano Ciurli wrote:
I'm not sure what you're suggesting--a real simultaneous fit, which somehow weights the fit in R-space and k-space to arrive at a single set of parameters? Or just two fits in series so that you only hit one button?
first of all let me tell you that the idea of running the fit in R-space somehow disturbs my simple mind, considering that the R-space depends on the processing parameters (window type and width, weight etc). So, I would assume that a fit in k-space would be auspicable. Then, I discovered that EXCURV, the competing program at least for biological XAS, performs the fit BOTH in R-space and in k-space, and I am pretty sure that the parameters it gets are slightly different.
then, yes, I was thinking of two different fits hitting the same button (let's say in Diana - or Artemis - instead of having just the option of running the fit either in R- or in k-space, one could have an additional button telling the program to run both fits)
My US$0.02 worth: Conceptually, corefining in k and R space is similar to corefining multiple k-weights. That is, it is corefining two different perspectives on the same data set, thus adding more points to the evaluation of chi-square without changing the information content of the data. The multiple k weight fit provides a clear benefit. The different k-weightings affect different parameters differently. k^3 enhances, in a sense, the sensitivity of the fit to delta_r relative to delta_e0 while k^1 does the reverse. Doing a 1,3 fit is a sort of compromise in hopes of partially disentangling the correlations between those parameters. I don't see the similar advantage to doing a k,R fit. That is not to say there isn't one, its just says that the merit of doing so has not yet percolated into my tiny brain. I just don't see how a k,R fit will serve to disambiguate some part of the fit that is ambiguous in a k-only or R-only fit. B P.S. I would be surprised if the choice of k-space window function has an effect on the fitting parameters outside their uncertainties so long as a modest value for dk is used. -- Bruce Ravel ----------------------------------- bravel@anl.gov -or- ravel@phys.washington.edu *** My cell phone number has changed. Please ask if you need the new number Environmental Research Division, Building 203, Room E-165 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/