[GSAS-II] Can Parameters be restricted?

[William Boyer]

If your data is noisy, limits can also help keep the parameters from getting stuck in physically impossible local minima.  For example, I might set a minimum FWHM to a little less than the narrowest peak my instrument can produce, then a few random points that happen to be above average cannot be mistaken for a peak.

 

Best regards,

William Boyer

 

From: gsas-ii-bounces at aps.anl.gov [mailto:gsas-ii-bounces at aps.anl.gov] On Behalf Of Toby, Brian H.
Sent: Monday, September 12, 2016 5:42 AM
To: Ivo Alxneit <ivo.alxneit at psi.ch>
Cc: Von Dreele, Robert B. <vondreele at aps.anl.gov>; gsas-ii at mailman.aps.anl.gov
Subject: Re: [GSAS-II] Can Parameters be restricted?

 

No.  

 

To be honest I am not sure why one would want to do this. When a parameter refines "out of range” it tells you something about your data or model. With a restraint, you would never know that for example, a Uiso value is refining negative by > 2 sigma or a occupancy is >>1. It makes sense to me to define a model so that the sum of occupancies is fixed — a well defined assumption. Defining a range for a parameter scares me as I could never know that my model is highly flawed is some way. 

 

Brian

 

On Sep 12, 2016, at 10:27 AM, Ivo Alxneit <ivo.alxneit at psi.ch <mailto:ivo.alxneit at psi.ch> > wrote:

 

Dear all

is there a way to restrict fitted parameters to a certain range? For
example restrict site occupancies to positive numbers or to a range such
as 0.9-1.0?

thanks for any help
-- 
Dr. Ivo Alxneit
Catalysis for Energy Group
Bioenergy and Catalysis Laboratory        phone: +41 56 310 4092
Paul Scherrer Institute                     fax: +41 56 310 2688
CH-5232 Villigen                      gnupg key: 0x515E30C7
Switzerland
https://www.psi.ch/ceg/catalysis-for-energy-group/

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