[GSAS-II] Clarification on using GSAS 2

[Von Dreele, Robert B.]

Nithya,
“Chi^2” is the sum of W*(Yo-Yc)^2; this is the thing that is minimized by the least squares and is a large number. It is roughly equal to the number of observations in a well refined system. Many folks mistakenly call chi^2/(Nobs-Nvar) as “chi^2”; that is really “reduced chi^2”. GOF = sqrt(reduced chi^2).
Best,
Bob

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From: nithya at igcar.gov.in<mailto:nithya at igcar.gov.in>
Sent: Wednesday, April 21, 2021 4:30 AM
To: Von Dreele, Robert B.<mailto:vondreele at anl.gov>
Subject: Re: [GSAS-II] Clarification on using GSAS 2

Dear Bob,

        Thanks for a prompt reply. I am still confused why the value of Chi2 is so high? Is this parameter equivalent to the one (of the R-factors) we get in the refinement performed using GSAS EXPGUI? Alos I would like to find out what all the R parameters that should be given for publishing in journals?

        Right now I am using GSAS-II revision 4408 (SVN version 4408). How can get updates of GSAS II in my computer?

with regards,
Nithya

From: Von Dreele, Robert B.
Sent: Tuesday, April 20, 2021 6:36 PM
To: nithya at igcar.gov.in ; Toby, Brian H.
Subject: RE: [GSAS-II] Clarification on using GSAS 2

Dear Nithya,
Given that your GOF is close to 1 (=1.53), you won’t do any better. If the difference curve reveals no remaining peak shape, position, intensity issues, then you are done. The high Rwp is probably because you didn’t count long enough to get a clean background (that’s where a good part of Rwp may come from).
The Ru+5 & Co+4 oxidation states exist as an electron counting requirement; the actual electron distribution is much closer to neutral atoms so one ought to use those for all atoms. In any event, the form factors for any element & its various valence states are essentially identical for most of the diffraction pattern above say 10deg 2-theta for Cu Ka radiation. My advice is to use zero valent atoms for all atoms irrespective of the apparent chemical valence.
Best,
Bob

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From: Nithya via GSAS-II
Sent: Tuesday, April 20, 2021 3:42 AM
To: gsas-ii at aps.anl.gov
Cc: Nithya R.
Subject: [GSAS-II] Clarification on using GSAS 2

Hi,

        This is Dr. Nithya Ravindran from India. I am using GSAS II to extract structural information of double perovskites and I have a couple of doubts to be clarified. They are listed below:

1. After a few iterations of refinement, a stable wR on 1399 observations 15.572% is obtained. Not able to refine further to reduce this value. Visual inspection of the plot of experimental and computed powder diffraction profile agrees well. Can this value be considered acceptable to rely on structural parameters?  We get Chi2 value very high (as high as 3213.91) and GOF 1.53; and RF AND RF2 4.958% AND 9.369% respectively on 12 reflections.  If not, what should be the minimum value of wR for a good fit?
2. in two of the ruthenium and cobalt based double perovskites, Ru and Co exist in +5 and +4 oxidation states respectively. Unable to select these ions as they are not included in the program. Please clarify.

Thank you,
With warm regards,
Nithya


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