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There is a generalized restraint where you could write your own function that will “push” your Frac values to be positive, but this would not be a good idea. If your Frac value(s) are negative by more than 2 sigma, then something is wrong. A value within 2
sigma of zero is experimentally indistinguishable from zero.
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<div class="">Usually this happens when you have site sharing and the refinement is trying to put more scattering density that 100% of the “heavy” atom or too little and the density is less than that of the “light” atom. In the “heavy” case are you sure about
the site assignments? In the “light" case could you have vacancies? It is also possible that your data are insensitive to the scattering from that site and any value for the occupancy gives about the same fit. It is also possible your model is very far from
correct and the model is not good enough for occupancies to be refined. It is perfectly reasonable to set the occupancy as 100% of one atom type and not refine that. Look to see if the Uiso value is reasonable. </div>
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<div class="">If you are refining the occupancy of for a single atom that does not share a site and the occupancy goes below zero then with x-ray data you either have a serious problem with some other part of your model or there is no scattering density in
that site and you should take that atom site out. With neutrons also consider an atom type with the opposite scattering phase (negative b). </div>
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<div class="">In any case, a constraint would serve to hide that something is wrong with your result. A restraint would make it harder to do that, but still is not addressing the problem. </div>
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<div class="">Brian<br class="">
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<div class="">On Nov 19, 2019, at 10:17 AM, Hao Liu via GSAS-II <<a href="mailto:gsas-ii@aps.anl.gov" class="">gsas-ii@aps.anl.gov</a>> wrote:</div>
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<div dir="ltr" class="">Hi,
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Is there a way to constrain the atomic occupancy to be non-negative?<br clear="all" class="">
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<div class="">Thanks,</div>
<div class="">Hao</div>
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<div dir="ltr" class="">Hao Liu, Ph.D.
<div class="">Assistant Professor</div>
<div class="">Chemistry Department, <span style="letter-spacing:0.2px" class="">Binghamton University</span></div>
<div class="">4400 Vestal Parkway East, NY 13902</div>
<div class="">Tel: +1 (607) 777-4671 </div>
<div class=""><a href="http://chemiris.chem.binghamton.edu/LIU/liu.html" target="_blank" class="">Group website</a></div>
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