[GSAS-II] Can Parameters be restricted?

[Testino Andrea (PSI)]

Hello,

I would add that in case chemical analysis define a specific composition, constraint a compositional window in a certain moment of the refinement, would help to adjust other parameters more efficiently. Then, the constraint might be removed.
Anyway, non-negative values or the possibility to let the parameters confined in a window is, in my view, not complicated to implement and very useful in several refinemens,

Andrea Testing,

Paul Scherrer Institut

Il 12/set/2016 08:50 PM, William Boyer <WBoyer at protoxrd.com> ha scritto:
>
> 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.
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>
>
> Brian
>
>
>>
>> On Sep 12, 2016, at 10:27 AM, Ivo Alxneit <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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>
>
>
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