[Ifeffit] Histograms with Demeter -- meaning of fit values

Walter Fu wpfu at wustl.edu
Mon Jun 25 03:21:06 CDT 2012


Hi all,

I'm trying to do a preliminary fit to the first shell of an amorphous
material, using a histogram generated from diffraction data.  I've been
using a script similar to the one given here:

http://bruceravel.github.com/demeter/dpg/examples/histo.html

When I run my script, it creates an SS path for each histogram bin and then
performs the fit, as expected.  However, if I then query the fit object for
a summary, I get a list of fitted values back.  My question is, what is the
meaning of these values?  Do they refer to the fit as a whole, or
individual paths within the fit?  For instance, is the "delR" value the one
which, when applied uniformly to all of the paths generated, produces the
best fit?  Or is it merely the value calculated for the last-used path
(presumably the one corresponding to last bin in the histogram), which may
be different for the second-to-last path or for the first path?  I'm also a
bit confused as to how the "amp" variable (line 30 in the script from the
link given above) comes into play.  Is the reported value a single
multiplicative factor which is applied to the histogram-generated
amplitudes of all the paths, or is there a separate "amp" for each path,
with only the one corresponding to the last-accessed path being reported?

I've tried looking through the Demeter source to get the answers, but given
my limited knowledge of Perl, my search is proving less than conclusive.
Any explanations you could give would be greatly appreciated.

-Walter
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20120625/71d49a2d/attachment.html>


More information about the Ifeffit mailing list