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[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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