[GSAS-II] GSAS 2 inquire

[Salasin, John]

Wei,

See responses below in Orange.

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John Robert Salasin
PhD Student
Materials Science and Engineering
University of Tennessee
302-766-0866<tel:302-766-0866> | Jsalasin at vols.utk.edu<mailto:Jsalasin at vols.utk.edu>
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From: gsas-ii-bounces at aps.anl.gov [mailto:gsas-ii-bounces at aps.anl.gov] On Behalf Of Wei Liu
Sent: Wednesday, June 14, 2017 8:55 PM
To: gsas-ii at aps.anl.gov
Subject: [GSAS-II] GSAS 2 inquire

Hi,
I am a new user of GSAS 2. I am trying to use GSAS 2 by reading the 'Help for GSAS-II'. However, I have some doubts, which cannot be found in this web page. Could someone advice on my questions below? Thanks a lot.
1. In the 'Unit Cells List', what is the 'Max Nc/Nobs 4 Start Volume 25 Use M20/(X20+1)?' What is the function of the refinement to this parameter? If I recall correctly this is used when trying to determine an unknown crystal structure. The program will fit the peaks locations to all possible crystal structure and use that function above as a weighting variable to say which is more likely.
2. In the 'Phase Data'/ 'Atoms' tab, for each of the atoms, there are refinement choices, 'X, XU, U, F, FX, FXU, FU'. May I know what is the difference between these choices? X will refine atom positions, U will refine their thermal parameters, F will refine they fractional occupancies. XU, FX, FXU, FU are just a combination of the above. If you do XU it will refine the thermals and site positions, and if you do FXU it will refine the positions, thermals, and occupancies.
Btw, may I know whether there is a more detailed manual of GSAS 2 besides the 'Help for GSAS-II'? This has not been developed as the program is still being developed. A lot of the math is the same from GSAS I and the manual for it is well documented. Normally you can find the information you are looking for in that manual. http://www.ccp14.ac.uk/ccp/ccp14/ftp-mirror/gsas/public/gsas/manual/GSASManual.pdf

And if a good reference document or book could be recommended for the procedure of using GSAS 2 and the theory (including those important parameters to refine in a normal neutron powder diffraction pattern) for the refinement of a neutron diffraction pattern, it will be very helpful as well. I would follow all the tutorials on the GSAS II website and do a lit review on the Rietveld refinement process as a whole. Research gate also has some good information.
Thank you very much for your help in advance,
Best regards,
Liu Wei
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