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RE: [sdpd] Trouble with GSAS tutorial on Sodium-para-hydroxybenzoate
Dear Yee Yan,
I don't know Dr Robert's tutorial, but evidently he is refining things
that you don't if he has 39 variables and you have only 11.
If you are in the Le Bail extraction step, POWPREF should not be run in
the middle of the process. Because the POWPREF run resets all observed
intensities to zero and during the first cycle GENLES will use those of
the dumb structural model (generally one H atom at a general position
x,y,z), this will make the next GENLES run to start form the wrong
point. If you are obliged to modify (by hand) the cell parameters, the
profile function parameter or the profile function itself. Then POWPREF
should be run, and several cycles of GENLES with 0 cycles should be run.
You will observe that the R factors decrease although you are using 0
cycles!. Go on with this 0 cycle procedure until you get an R factor
comparable with the one you were attaining after running POWPREF.
You should also look at the Rietveld archives or the CCP14 site for more
tutorials. And I suggest you to look at Brian Toby's EXPGUI web site
where there are several hints for correctly using GSAS/EXPGUI suite.
Good luck and have fun.
Leopoldo
Dr. Leopoldo Suescun
Laboratorio de Cristalografía, Estado Sólido y Materiales
(Cryssmat)/Dequifim
Facultad de Química. CC 1157
Av. Gral. Flores 2124
11800 Montevideo, Uruguay
http://cryssmat.fq.edu.uy/leopoldo/leo.htm
-----Mensaje original-----
De: Tay Yee Yan [mailto:one_twinklestar...@yahoo.com.sg]
Enviado el: viernes, 08 de abril de 2005 3:23
Para: sdpd...@yahoogroups.com
Asunto: [sdpd] Trouble with GSAS tutorial on Sodium-para-hydroxybenzoate
Hi All:
I am currently trying on Dr Robert's tutorial on
Rietveld Refinement of the Crystal Structure of
Sodium-para-hydroxybenzoate.
http://www.uni-bayreuth.de/departments/crystal/rietveld/
the first part on the tutorial is on extracting the
intensities using Le Bail refinement. This part of
refinement is fine. However, when I switch back to
Rietveld refinement using rigid body where I follow
very "closely" to the tutorial, it seem that my
refinement doesn't seem to converged when I am
refining the overall temperature factor. I am getting
Rwp of 0.5583 which is extremely large compared to the
figure shown on page 48 of the tutorial which shows a
Rwp of 0.0715. A noticable difference is the number of
variables used. I found mine is only 11 but there are
39 variables refined as shown in the tutorial. I
repeated many times but giving the same answer.
1) Even if I follow 'closely' to the tutorial, did I
still miss out anything? My apology because I just
started trying out GSAS
2) Secondly perhaps some other people would have post
this question which is on the appropriate use of
powpref. Is is mentioned that if the profiles are
altered manually by hand, powpref should be run. I
notice that there are times where i didn't alter
manually and if I still run powpref, the refinement
would crash. Does it mean that powpref cannot be run
often even though if the profiles are not altered
manually?
Thanks in advanced first!!!!
Regards,
Yee Yan
School of Materials Science and Engineering
Nanyang Technological University
Singapore
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