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[sdpd] Re: : Are restraints as good as observations ?

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      <p>&gt;No such restraint on posting to the SDPD mailing list.<br>
<br>
Well, I see that there are cases where &quot;Even Armel Le Bail&quot; doesn&#39;t accept restraints ;-)<br>
<br>
&gt;Just to explain how restraints can be useful<br>
&gt;even at the SDPD step...<br>
<br>
No doubt that at the initial SDPD stages restraints as well as other tricks may be useful for the convergence stabilization. Being left at the final stage, however, they devalue the initial efforts leaving the researcher without any means for the solution validation.<br>
Hypothetically, restraints might be made more-or-less robust if:<br>
1) A certified generally accepted method for choosing safe weights of restraints is developed.<br>
2) The uncertainties are derived from a separately calculated variance-covariance matrix free of restraints.<br>
Since neither 1) nor 2) are available currently, I don&#39;t see other real alternatives for the final SDPD stage but free refinement with minimal rigid constraints.<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>, xtal &lt;xtal...@...&gt; wrote:<br>
&gt;<br>
&gt; Hi,<br>
&gt; <br>
&gt; Just to explain how restraints can be useful even at the SDPD step. The <br>
&gt; real space approach for solving the K4P2O7 structure at room temperature <br>
&gt; needed 3 independent P2O7 groups, each with a torsion angle, and a long <br>
&gt; list of K atoms (12). It was not successful. Then, considering that the <br>
&gt; room temperature phase was obviously related to the 300°C phase (Z=36 <br>
&gt; instead of Z=4, both hexagonal), the high temperature small cell structure <br>
&gt; was extended in the large cell as a starting model for a Rietveld <br>
&gt; refinement. The process converged extremely slowly (&gt;1000 cycles with <br>
&gt; damping) only if restraints on the P2O7 groups were used, otherwise, the <br>
&gt; refinement exploded immediately. So, this is one of the rare cases where <br>
&gt; saying &quot;solved by the Rietveld method&quot; is true - with restraints.<br>
&gt; <br>
&gt; Best,<br>
&gt; <br>
&gt; Armel<br>
&gt;<br>
<br>
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