On Tuesday 05 July 2005 18:39, Alison Costello wrote:
I have a question regarding the calculation of the R-factor. I am currently using SIXPACK to process my protein EXAFS data, and I fit the data in both k-space and in q-space (back-transformed k-space). I typically will use k-ranges = [1-13.6] and for multiple-scattering fits, will use an R-range of 0.1 - 4.5. Sometimes, the R-factor for the q-space fit is greater than the R-factor for the k-space fit, which should not be true, as the back-transformed space filters out noise. I am confused as to why this occurs, and am wondering if the R-factor for the back-transformed space is calculated differently than for k-space fits.
Alison, I would expect the R factor to be bigger in k space due to the high frequency portions of the data that get filtered out of the q space data. That is what happened in the one example I just tried. What are some example values of the R-factors? Do they differ by about a factor of 2? One thing that occurs to me is that chi(q) is a complex function while chi(k) is a real function. In Ifeffit those two functions should have the same number of points, but the R factor in q is computed using both parts of the complex function. Thus it would seem reasonable if they differed by about 2 and there were no significant Fourier components in the data beyond your rmax. B -- Bruce Ravel ----------------------------------- bravel@anl.gov -or- ravel@phys.washington.edu Environmental Research Division, Building 203, Room E-165 Argonne National Laboratory phone: (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/