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[sdpd] Re: Hydrogen atoms and SDPD from X-ray data
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<p>Dear Armel,<br>
<br>
It would be more rational to consider H-atoms not as a curse and a plague, but as one of those rare and thus precious indicators of the structure refinement quality. If I am able to refine H-atom positions without restraints obtaining reasonable geometry I may be sure that the data and the structure model are of good quality. With the development of nowaday laboratory XRPD instruments and methods it is becoming more and more realistic.<br>
<br>
Best regards,<br>
Leonid<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 <armel.le_bail...@...> wrote:<br>
><br>
> Hi,<br>
> <br>
> A recurrent subject... Hydrogen atoms are a kind of curse to<br>
> conventional X-ray Powder Diffraction SDPD experts.<br>
> <br>
> No kidding, a real plague.<br>
> <br>
> With or without them, in general you can obtain a good fit.<br>
> <br>
> You rarely can see them on a Fourier difference map.<br>
> <br>
> If you try to publish without them, invariably a referee requires firmly<br>
> that you make more efforts.<br>
> <br>
> If you publish with them, they will be generally guessed at highly<br>
> probable positions by many software, eventually optimized, but when<br>
> inserted into the Rietveld refinement, even at fixed positions, then<br>
> the positions of the other atoms will move a bit, and their distances<br>
> with the H atoms will change if compared with your optimization.<br>
> Then you may have to introduce constraints or restaints or rigid body<br>
> and so on, then you may want to go back to your optimization,<br>
> then back to the Rietveld fit, then ....<br>
> <br>
> A curse...<br>
> <br>
> But a complete hydrogen bonding scheme is so nice.<br>
> Anyway, you cannot discuss it too much since the H atoms were<br>
> guessed and more or less fixed, etc.<br>
> <br>
> A plague...<br>
> <br>
> If one proposes a one-button to press process incorporated into<br>
> a Rietveld software, this would be quite nice (Lachlan would agree<br>
> probably with that query to the developers).<br>
> <br>
> Best wishes,<br>
> <br>
> Armel<br>
><br>
<br>
</p>
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