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


Am 12.09.2016 um 11:42 schrieb Toby, Brian H.:

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.


On Sep 12, 2016, at 10:27 AM, Ivo Alxneit < <>> 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

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