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GSAS Functions In Topas
apw247 #1
User title: Yuanpeng
Member since Apr 2019 · 3 posts · Location: National Institute of Standards and Technology
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Subject: GSAS Functions In Topas
Dear Topas fellows:

When I tried to do the Rietveld refinement for Si standard sample with both GSAS-I and Topas using the same peak profile function (TOF_PV convoluted with GSAS back-to-back), I couldn't reproduce the results in between them. Especially in the high-Q region, the result from Topas is broader than that from GSAS-I.

Here following is the macro I used in Topas, trying to reproduce the GSAS type-3 function:

"""
TOF_PV
macro GSAS_btb       'GSAS back to back function type 3, GSAS manual page 150
{
   push_peak
       exp_conv_const = alpha0 + alpha1 / D_spacing;
   bring_2nd_peak_to_top
      exp_conv_const = beta0 + beta1 / D_spacing^4;
   add_pop_1st_2nd_peak
}
"""

After all, what I want to achieve is: if I grab peak profile parameters in GSAS-I (say, using the GSAS type-3 function) and put them in Topas, when I run 0 iteration calculation, I will get identical calculation result from both. Can anyone provide some guidance on how to write macros in Topas to achieve this? Many thanks!

Cheers,
Yuanpeng
AlanCoelho #2
Member since Aug 2011 · 280 posts
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Hi Yuanpeng

I will have a look; can you send me by e-mail your INP and data file please.
alan
apw247 #3
User title: Yuanpeng
Member since Apr 2019 · 3 posts · Location: National Institute of Standards and Technology
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Thanks a lot Alan! I will send you the files!

Cheers,
Yuanpeng
AlanCoelho #4
Member since Aug 2011 · 280 posts
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I think the peak shape as described in test_examples\tof_bank2_1.inp should be used; ie.

Replace the following:

   GSAS_btb

with

   TOF_Exponential(a0, 100, ,0, 1, difc_b5, -)
   TOF_Exponential(b0, 100, b1, 100, 4, difc_b5, +)
apw247 #5
User title: Yuanpeng
Member since Apr 2019 · 3 posts · Location: National Institute of Standards and Technology
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Hi Alan,

Thank you so much for pointing that out!

Cheers,
Yuanpeng
johnsoevans (Administrator) #6
User title: John Evans
Member since Aug 2009 · 209 posts
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Alan: it's a long time ago, but my recollection was the profile functions gsas vs topas were essentially identical but I didn't think there was a direct correspondence between the actual numerical values of the coefficients between topas and gsas.  I think Yuanpeng wanted exact equivalence (I'm guessing to tie in with rmcprofile?).
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