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[sdpd] Re: Restraints and so on

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      <p>&gt;we can solve structures much more complex than those we can refine &gt;without restraints.<br>
<br>
If a minimum of a cost function is found for a constrained structure model in direct space, but the unconstrained refinement is impossible, one does not have criteria (e.g. distances, angles, their e.s.u. etc.) to assert that the structure is determined or soled. For such cases a special term should be proposed instead of structure solution, say, structure optimization. <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>
--- In <a href="mailto:sdpd%40yahoogroups.com";>sdpd...@yahoogroups.com</a>, Armel Le Bail &lt;armel.le_bail...@...&gt; wrote:<br>
&gt;<br>
&gt; <br>
&gt; <br>
&gt; &gt; &gt;People are inclined to see what they want to see. Restraints <br>
&gt; &gt; obscure the accuracy and validity of the refined parameters.<br>
&gt; &gt;<br>
&gt; &gt;It is a double-edge sword:-) I got it.<br>
&gt; <br>
&gt; To be a powder diffractionist is more and more difficult, especially when<br>
&gt; the data/parameters ratio (d/p) becomes small. With d/p &gt;20, this is<br>
&gt; generally comfortable though not ensuring always to locate so well the<br>
&gt; light atoms. With d/p &lt; 3 this is dramatic. And between 3 and 20 this is<br>
&gt; very frequently problematic ;-). We face such problems because of<br>
&gt; the high performance of structure solution in direct space : we can solve<br>
&gt; structures much more complex than those we can refine without<br>
&gt; restraints. Approximate limits were proposed :<br>
&gt; <a href="http://www.cristal.org/iniref/aca97/t18.gif";>http://www.cristal.org/iniref/aca97/t18.gif</a><br>
&gt; <a href="http://www.cristal.org/iniref/aca97/aca97.html";>http://www.cristal.org/iniref/aca97/aca97.html</a><br>
&gt; <br>
&gt; If referees admitted arguments like &quot;well, interatomic distances and<br>
&gt; angles look out of expected values, but this is powder diffraction you know,&quot;<br>
&gt; then you would not have to use restraints, constraints, rigid body, if not<br>
&gt; quantum chemical optimization. But they do not admit the argument.<br>
&gt; If you wish, you may give the results with and without the restraints (etc),<br>
&gt; so that everybody is happy (well, sometimes, without restraints it may go<br>
&gt; completely crazy...). After all, many journals propose the storage<br>
&gt; for supplementary material.<br>
&gt; <br>
&gt; Best,<br>
&gt; <br>
&gt; Armel<br>
&gt; <br>
&gt; PS -<br>
&gt; d = number of reflections reduced by some overlaping weight depending<br>
&gt;        on the FWHM<br>
&gt; p = number of independent atomic coordinates<br>
&gt;<br>
<br>
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