Matt,

You are correct and In GSAS 'Fitted" is smaller than "- Bknd" as it should
be. If it is not, something is wrong.

 

Peter Zavalij 



X-ray Crystallographic Center
University of Maryland

College Park, MD



Office: (301)405-1861
Lab: (301)405-3230

Fax: (301)314-9121



 

 

 

From: Beekman, Matthew [mailto:mbeek...@cas.usf.edu] 
Sent: Saturday, March 07, 2009 7:20 PM
To: Peter Y. Zavalij; rietveld_l@ill.fr
Subject: RE: cRs

 

Hello all,

 

A quick question regarding the background subtracted R values:

 

In the .LST file, GSAS gives two sets of values for wRp and Rp: "Fitted" and
"- Bknd." I understood the Fitted values to be calculated including the
background, and the -Bknd to be calculated with the background removed from
both the calculated and observed patterns. Is this correct? 

 

I notice in some refinements I am doing that the - Bknd values are smaller
than the Fitted values which is not what I would have expected.

 

Thanks in advance,

 

Matt

 

------------------------------------------------------------------
Matt Beekman
Department of Physics
University of South Florida
4202 East Fowler Ave., PHY 114
Tampa, FL 33620
Phone: (813) 974-8236
Fax: (813) 974-5813
Email: mbeek...@cas.usf.edu
Visit the Novel Materials Laboratory Website:
http://chuma.cas.usf.edu/~gnolas/

 

  _____  

From: Peter Y. Zavalij [mailto:pzava...@umd.edu]
Sent: Wed 3/4/2009 9:18 AM
To: rietveld_l@ill.fr
Subject: RE: cRs

There are others important criteria of refinement quality implemented in
many software but rarely analyzed or published. These are Rp' and Rwp'
calculated with background subtracted from both observed and calculated
data. They are especially important when dealing with high background cases,
e.g. Fe compounds measured using Cu radiation or H compounds using neutrons.

These R's are always higher  but more realistically showing the quality of
profile fit. I saw neutron refinements with Rwp<10% and completely wrong
phase fitted. Yes! R(Bragg) was near 100% so were Rp' and Rwp' but Rp and
Rwp were so good.

 

Peter Zavalij 

X-ray Crystallographic Center
University of Maryland

College Park, MD

Office: (301)405-1861
Lab: (301)405-3230

Fax: (301)314-9121

 

From: Whitfield, Pamela [mailto:pamela.whitfi...@nrc-cnrc.gc.ca] 
Sent: Wednesday, March 04, 2009 8:17 AM
To: Jon Wright; olga.smirn...@hw7.ecs.kyoto-u.ac.jp
Cc: rietveld_l@ill.fr
Subject: RE: cRs

 

I'll put my tuppence worth in here as there can be another consideration
with high backgrounds.

 

Normally I wouldn't touch the background, but there are times when a high
background causes an artificially shallow minimum and the refinement isn't
going very far (Rietveld software is usually minimizing Rwp so starting out
with a low Rwp isn't very good).  

In this instance making the Rwp worse when the peaks aren't fitting can
help.  As a rule, before doing that I'd switch to xye format to preserve the
statistics before doing a linear, across-the-board count substraction (not
attempting to take out any wiggles which will distort peak shapes, etc -
leave that to the background modelling).  I suppose you may introduce some
slight distortions into the result but if the refinement is going nowhere
then....

 

However if you try this approach your final Rwp may end up significantly
worse than other papers in the literature, and there are occasions when
referees can get hung up on residuals when a difference plot speaks a
thousand words (or numbers).  I suppose you could always go back and fit the
refined model to the original data to get a wonderful, if slightly
meaningless Rwp for the paper!

 

Pam

 

  _____  

From: Jon Wright [mailto:wri...@esrf.fr]
Sent: Wed 04/03/2009 4:00 AM
To: olga.smirn...@hw7.ecs.kyoto-u.ac.jp
Cc: rietveld_l@ill.fr
Subject: Re: cRs

Dear Olga,

As you've noticed, the Rietveld R-factors tell you how well you fit the
pattern, but don't tell you how well you fit the peaks. There is a
correlated intensities R-factor which follows from the methods outlined in:

J. Appl. Cryst. (2004). 37, 621-628    [ doi:10.1107/S0021889804013184 ]
"On the equivalence of the Rietveld method and the correlated integrated
intensities method in powder diffraction"
W. I. F. David

If I remember well; to get the corresponding Rfactor you just take the
correlated intensities chi^2 for your model and divide by the same
statistic with all I_calc set to zero and perhaps convert to a
percentage. (Hopefully Bill is reading and will correct that if I've
screwed it up).

Fortunately any statistics which rely on peak intensities are open to
abuse by using the wrong cell or space group etc, hence the value of
Rietveld Rfactors and a really high background ;-)

Best,

Jon




Olga Smirnova wrote:
> Dear All,
>
> How is life with conventional R factors when you always have to divide
> by zero background?
> Let's have time. Considering a part of the profile without peaks one
> gets 100% cR.
> I did not give the agreement factor; I would say those cR with all
> non-excluded points is incorrect, but cR for
> points with Bragg contribution is almost the same!
> Do you decrease the Rs by adding the background or do you increase cRs
> by subtracting the background?
>
> OS
>
> PS I did not ask my supervisor before sending such a mail.
>
>

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