Hi Arianna,

The instrument broadening parameters are unique to each instrument. There
was a paper published that looks at a few common instruments (Kaduk, J. and
Reid, J. Powder Diffraction 26 (1) 2011, 88-93) and reports U,V,W,X,Y,
asym, S/L, H/L, etc. These may be a good guideline, but your instrument
should really be calibrated with a SRM.

I had a similar result to you on my instrument when refining the instrument
parameters using LaB6 SRM. I achieved better fits refining first X and Y,
then UVW. I think if you get large positive or negative values for these it
may indicate that there is not much broadening in the Caglioti type
equation, and the parameters diverge in the fit. You could play around with
some dummy instrument parameters and make simulated histograms to see the

If you do not have a standard reference, you really can't deconvolute the
instrument and sample effects.

As for the difference between old and new GSAS, I would take a look in the
source code, it is well commented as to what models have been incorporated
and their references.

Hope this helps,

On Tue, Feb 27, 2018 at 10:00 AM, <gsas-ii-requ...@aps.anl.gov> wrote:

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> Message: 1
> Date: Tue, 27 Feb 2018 16:03:36 +0000
> From: Arianna Lanza <arianna.la...@iit.it>
> To: "gsas-ii@mailman.aps.anl.gov" <gsas-ii@mailman.aps.anl.gov>
> Subject: [GSAS-II] instrument parameters
> Message-ID: <a06a772caf334efaaa50f35772a46...@iit.it>
> Content-Type: text/plain; charset="iso-8859-1"
> Dear all,
> I am new to GSAS II and have no experience with old-GSAS, so forgive me
> for the basic level of the questions.
> I don't fully understand the meaning of the parameters describing the
> instrumental profile.
> ?         What is Z? I could not find it in the old-GSAS manual.
> ?         How can I judge if the U V W X Y Z are reasonable?  (in case I
> don't have a reference profile from a standard) And how should a well
> behaved peak width plot look like?
> ?         I read that U W X Y are constrained to be >= 0, while V should
> be <= 0. However I have a not so badly-fitted plot where U refined to very
> negative, V to very positive, W is small and negative, X is positive and Y
> negative. So (i) those parameters have no constraint by default!Why not?
> Can I do it with equations?; (ii) even from my na?ve understanding it looks
> like a nonsense. Most likely it is because I have broad peaks and the
> interplay between strain/size/etc and instrumental parameters converged to
> this artificial mess. Is it possible to deconvolute the instrument
> contribution from the sample contribution, in absence of a standard?
> ?         SH/L is one unique parameter as opposed to 2 separate parameters
> in old-GSAS, what is its meaning now, the units and/or reasonable range?
> If you can help with even partial answers or point out useful sources of
> information, I will be grateful!
> Kind regards,
> Arianna
> ,_,.-'???'-.,_,.-'???'-.,_,.-'???'-.,_,,.-'???'-.,_,.-'???'-.,_,
> Arianna Lanza, PhD
> Center for Nanotechnology Innovation@NEST
> Istituto Italiano di Tecnologia
> Piazza San Silvestro, 12, 56127, PISA, Italy
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Nick Weadock
PhD Candidate, Materials Science
California Institute of Technology
1200 E California Blvd, Pasadena, CA 91125
MC 138-78
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