[Ifeffit] Fit in R and k space

Matt Newville newville at cars.uchicago.edu
Tue Jul 12 23:04:02 CDT 2005

Hi Stefano,

On Fri, 8 Jul 2005, Stefano Ciurli wrote:

> Hi Bruce and Matt,
> just thinking while fitting: wouldn't it be 
> possible-better-auspicable-desirable to have a simultaneous fit in 
> both R and k space and evaluate the statistics of the two fits in 
> some intelligent way?
> Just a though
> Stefano

Trying fits that simultaneously uses R- and Q-space is an OK
idea, and like Bruce says, isn't very different from using
multiple k-weights.  I think it would be really interesting to
fit in the "R+Q" space generated by the wavelet transforms.  

For now, I sort of doubt that fitting in R-space and Q-space
simultaneoulsy will make a huge difference to fitting in R-space
alone, but I'd be curious to see evidence either way.

On the other hand, fitting unfiltered k-space is always a bad
idea. Using R-space and Q-space allows you to select the k- and
R-range of your data to model, which you want.  This also allows
a proper error analysis.  Using unfiltered k-space does not
allow this as there are systematic signals (the higher shells)  
in the data that are not modelled.  Fitting in unfiltered
k-space also gives erroneous R-factors, as was recently
discussed here. 



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