[Ifeffit] Fit in R and k space

Bruce Ravel ravel at phys.washington.edu
Fri Sep 16 21:46:41 CDT 2005

On Monday 19 September 2005 07:31, Stefano Ciurli wrote:
> Hi Matt,
> Thinking about problems. You once told me that:
> >   Fitting in unfiltered
> >k-space also gives erroneous R-factors, as was recently
> >discussed here.
> would you give me a direction to the discussion about this point that
> I can read?


I don't know if there is an explicit reference.  Just consider the functional 
form of the R-factor.  It's essentially a measure of misfit -- a quantitative 
measure of how much the red line falls on top of the blue line.  Consider a 
first shell fit to copper metal.  

In R space and under the first peak, the red line falls right on top of the 
blue line and the R-factor is small.  

In k space, the red line only has the frequencies of the first shell while the 
data has all frequencies.  In this space, the red line doesn't look much like 
the blue line, even for an excellent fit.  Consequently the R factor is quite 

That is, I think, what Matt meant by "erroneous" -- not that I can claim to 
speak for him! ;-)


 Bruce Ravel  ----------------------------------- bravel at anl.gov  -or-
                                                  ravel at phys.washington.edu
 Environmental Research Division, Bldg 203, Room E165
 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 
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