Dear Brian,
if I may to drop the comment. The enquiry is not entirely without merit.
It is not infrequent that one combines information from many sources to
build a model and constrains help to fix deficiency of a single method.
I guess that one case where a hard limit could be useful is a case of
vacancies in non-stoichiometric compounds. As you say it is easy to
constrain a site shared by multiple elements if they add up to 1. Yet,
there has been always a problem, also for GSAS I, to deal with a sum of
vacancy + element on a site. From this what I observed the refinement
was trying to compensate vacancies with artificiality increased thermal
displacement parameters if the data at higher 2-Theta were not "perfect"
(actually only a case of good TOF with neutrons or an exceptional
quality X-ray diffraction). If I know that the occupancy cannot be
larger than e.g. 1 for a site from other sources (e.g. spectroscopy or
other x-ray neutron diffraction experiments) then it should be possible
to set a hard limit on the site. Of course such decision must be well
substantiated. Otherwise one risks producing some rubbish results...
Best wishes
Andrzej
Am 12.09.2016 um 11:42 schrieb Toby, Brian H.:
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.alxn...@psi.ch
<mailto:ivo.alxn...@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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Advanced Photon Source Chief Computational Scientist
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Argonne, IL 60439-4856 e-mail: brian dot toby at anl dot gov
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--
Dr. Andrzej Falenty <afal...@uni-goettingen.de>
Abt. Kristallographie
GZG, Georg-August-Universität
Goldschmidtstraße 1
D-37077 Göttingen
Tel. 0049 551 3912546
http://kristall.uni-mki.gwdg.de/
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