[Ifeffit] R-factor

Bruce Ravel bravel at anl.gov
Wed Jul 6 09:43:03 CDT 2005

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.


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.


 Bruce Ravel  ----------------------------------- bravel at anl.gov  -or-
						  ravel at 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/

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