Thank you to ALL who have replied,
I forward here the main answers, so that some future GSAS-II user browsing the 
mail archive can find some info and does not need to bother you again for the 
same things.

Instead, I will probably bother you again for different things,

Kind regards,

Arianna Lanza

From: Nick Weadock []
Sent: martedì 27 febbraio 2018 19:39
To:; Arianna Lanza <>
Subject: Re: GSAS-II Digest, Vol 162, Issue 1

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 difference.

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,

From: Von Dreele, Robert B. []
Sent: martedì 27 febbraio 2018 17:40
To: Arianna Lanza <>
Cc: Toby, Brian H. <>
Subject: RE: instrument parameters

Dear Arianna,
‘Z’ is a constant Lorentzian broadening term (i.e. independent of 2-theta or 
TOF) recently added as it seems to be occasionally useful for some instrument 
characterization. As for the values, for conventional (especially neutron) 
diffractometers U & W are > 0 and V < 0 so the curve is a parabola with a 
minimum somewhere near 2-theta for the monochromator (see the garnet neutron 
tutorial example in GSAS-II). Lab x-ray diffractometers will be parabolic but 
usually fairly flat thus being a bit difficult to refine. Typical older ones 
have U~2, V~-2 and W~5. Newer ones with focusing optics will be different & 
smaller, however the Gaussian & Lorentzian resolution curves should be never 
negative.  GSAS-II will show the plots of the resolution curves for Gaussian, 
Lorentzian and their combination (pseudo-Voigt) as calculated and fitted – see 
Instrument Parameters. As Brian Toby noted, for GSAS-II the object is to 
calibrate your instrument with a standard (Si, LaB6, etc.) to get good values 
of U, V, W  (& X, Y – careful with these as sample broadening shows up in them 
which is NOT instrument broadening). Then never refine them again for the same 
instrument setup (same slits, etc.). Instead refine the sample broadening for 
each phase (see the Phase/Data tab for these) as mustrain & size.

From: Toby, Brian H. []
Sent: martedì 27 febbraio 2018 17:29
To: Arianna Lanza <>
Cc: Von Dreele, Robert B. <>
Subject: Re: [GSAS-II] instrument parameters


   We need to tabulate the equations we use for FWHM as a function of the 
profile variables somewhere (you could read the source code), but a quick 
answer is that you can see the resulting FWHM curves under the instrument 
parameters. Also, the new parameters (Z plus a bunch of TOF terms) are 
generally not needed for unless one is doing unusually careful fitting of an 

    First of all, I am assuming that you are fitting a standard such as NIST 
LaB6 that has minimal broadening. If not, you should do that before starting 
with a “real” sample. You don’t say but I will also assume you are using a lab 
instrument. A good place to start is to fit individual peaks across the range 
of your pattern so you can see how the Lorentzian (gamma) and Gaussian (sigma) 
widths vary without constraints. You can also see how well your UVW & XY curves 
reproduce this.


On Tue, Feb 27, 2018 at 10:00 AM, 
<<>> wrote:
Send GSAS-II mailing list submissions to<>

To subscribe or unsubscribe via the World Wide Web, visit
or, via email, send a message with subject or body 'help' to<>

You can reach the person managing the list at<>

When replying, please edit your Subject line so it is more specific
than "Re: Contents of GSAS-II digest..."

Today's Topics:

   1. instrument parameters (Arianna Lanza)


Message: 1
Date: Tue, 27 Feb 2018 16:03:36 +0000
From: Arianna Lanza <<>>
To: "<>" 
Subject: [GSAS-II] instrument parameters
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 Lanza, PhD

Center for Nanotechnology Innovation@NEST
Istituto Italiano di Tecnologia
Piazza San Silvestro, 12, 56127, PISA, Italy

-------------- next part --------------
An HTML attachment was scrubbed...


GSAS-II mailing list<>

End of GSAS-II Digest, Vol 162, Issue 1

Nick Weadock
PhD Candidate, Materials Science
California Institute of Technology
1200 E California Blvd, Pasadena, CA 91125
MC 138-78
GSAS-II mailing list

Reply via email to