[GSAS-II] About PDF in GSAS2

[Toby, Brian H.]

I am forwarding this to get Bob's reply into the archives.

I'd like to also add that the PDF routine in GSAS-II needs some more validation. It simplifies all the semi-rigorous corrections usually applied for x-ray PDFs into a single ad hoc correction value that can be varied with a slider to find the optimal. We would be interested in feedback on how this performs.

Begin forwarded message:

Shoaib,
Although Brian's remark is true - a PDF with CuKa radiations is quite poor - one can generate it quite simply in GSAS-II. The steps are:
1) Select 'Make new PDFs' from the main Calculate menu - this will make a new entry 'PDF' in the data tree.
2) Under 'PDF' select 'PDF Controls' and Add element for each element in your sample
3) For each element change the number to match the composition (could be # per cell) - a PDF will be computed automatically
4) Adjust the formula volume as needed and set other parameters as needed - each time the PDF will be updated.
5) Look at G(r) & F(Q); G(r) should have minimal features at G(0) & F(Q) should taper smoothly to zero at large Q adjust slider to best fit.
GSAS-II has no PDF modeling tools & the PDF generator is still under development, thus no tutorial for it.
Best,
Bob Von Dreele

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Brian H. Toby, Ph.D.                            office: 630-252-5488
Senior Physicist/Section Head for Scientific Software
Advanced Photon Source
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Argonne National Laboratory
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