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Re: [sdpd] Re: Are restraints as good as observations ?
- To: "rietveld_l...@ill.fr" <rietveld_l...@ill.fr>
- Subject: Re: [sdpd] Re: Are restraints as good as observations ?
- From: Leonid Solovyov <l_solovyov...@yahoo.com>
- Date: Tue, 30 Jul 2013 08:09:10 -0700 (PDT)
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<p><br>
<br>
> Anything<br>
can be used for good or/and bad, including restraints.<br>
<br>
Certainly,<br>
but when using restraints one can never be sure what is good or bad. One of the<br>
worst consequences of geometric restraints is that they incapacitate the<br>
structure geometry as a validation and accuracy indicator. In powder<br>
diffraction such indicators are so rare and precious that loosing one of them<br>
is a fishy price for good-looking (but stinky) molecules.<br>
<br>
>All one<br>
needs when using restraints is to know what<br>
> to<br>
restrain which implies knowledge of chemistry<br>
><br>
especially structural chemistry (or crystal-chemistry)<br>
> - you<br>
cannot find needle if you don't know what you<br>
> are<br>
searching for.<br>
<br>
The problem<br>
is not only in finding a needle (structure solution), but also in measuring<br>
its length (structure refinement). You can not measure a needle if you restrain<br>
its length.<br>
<br>
*******************************************************<br>
Leonid A. Solovyov<br>
Institute of Chemistry and Chemical Technology<br>
660049, K. Marx 42, Krasnoyarsk, Russia<br>
<a href="http://sites.google.com/site/solovyovleonid">http://sites.google.com/site/solovyovleonid</a><br>
*******************************************************<br>
<br>
________________________________<br>
From: Peter Y. Zavalij <<a href="mailto:pzavalij%40umd.edu">pzavalij...@umd.edu</a>><br>
To: Alan Hewat <<a href="mailto:alan.hewat%40neutronoptics.com">alan.hewat...@neutronoptics.com</a>>; "<a href="mailto:rietveld_l%40ill.fr">rietveld_l...@ill.fr</a>" <<a href="mailto:rietveld_l%40ill.fr">rietveld_l...@ill.fr</a>> <br>
Cc: Armel Le Bail <<a href="mailto:xtal%40noos.fr">xtal...@noos.fr</a>>; Leonid Solovyov <<a href="mailto:l_solovyov%40yahoo.com">l_solovyov...@yahoo.com</a>> <br>
Sent: Tuesday, July 30, 2013 9:14 PM<br>
Subject: RE: [sdpd] Re: Are restraints as good as observations ?<br>
<br>
<br>
Anything can be used for good or/and bad, including restraints.<br>
<br>
All one needs when using restraints is to know what to restrain which implies knowledge of chemistry especially structural chemistry (or crystal-chemistry) - you cannot find needle if you don't know what you are searching for.<br>
<br>
When working on one of my first structure (and quite complex one) (tma)Mo4O12 (it may be still on Armel's list of most complex structures so powder diffraction) I tried to restrain MoO6 octahedra to be "better". They all were quite distorted. However idealizing octahedra made Rp, RBragg, difference curve etc. much worse. After looking literature on other Mo oxides (I had to do it first) I realize that not all octahedra are ideal... so no restraints on octahedra geometry could be justified nor were necessary in this case.<br>
On the other hand in many/most cases it may be not possible to refined organic/pharmaceutical structures without restraining/constraining geometry of the molecules.<br>
<br>
About "patternless" Rietveld refinement: there is DLS software that allows to optimize geometry using restraints only in know unit cell. And it works... at least when I use it as pre-Rietveld structure optimization. It gave the same results as DFT optimization but in a second or two (not hours and days).<br>
<br>
Peter<br>
__________________________________<br>
Peter Zavalij<br>
X-ray Crystallographic Center<br>
University of Maryland<br>
College Park, MD 20742<br>
Phone/Fax: (301)405-1861<br>
http:/www.chem.umd.edu/crystallography<br>
<br>
[Non-text portions of this message have been removed]<br>
<br>
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