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[sdpd] Re: Dubious structure
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<p>Hi, Armel!<br>
<br>
I, personally, don't like constraints since I prefer being surprised to getting satisfied ;-). However, in our imperfect world constraints happen to be useful and even unavoidable sometimes.<br>
Publishing a separate table for unconstrained refinement might be reasonable but not always possible as in some cases the refinement may be unachievable without certain constraints (on H atoms, for instance). Commonly, it would be enough to follow a simple rule: if you constrain some structural parameters you don't make assumptions and conclusions based on these parameters in the publication. If all atomic coordinates are constrained or restrained (even softly) the whole structure can not be considered as being solved and refined.<br>
<br>
Best regards,<br>
Leonid<br>
<br>
--- In <a href="mailto:sdpd%40yahoogroups.com";>sdpd...@yahoogroups.<wbr>com</a>, Armel Le Bail <armel.le_bail...@<wbr>...> wrote:<br>
><br>
> Hi,<br>
> <br>
> The question today is : shall we use systematically<br>
> either soft constraints on interatomic distances or rigid<br>
> body refinements or ab initio quantum chemistry<br>
> calculations (or etc) when dealing with Rietveld refinements.<br>
> <br>
> The simple presentation of the raw result may generate<br>
> the comment "dubious structure" from the "usual monster<br>
> referee," even saying that "this is powder, low resolution<br>
> data (or intrinsic strong line broadening), no accuracy can<br>
> be expected on the atomic coordinates and interatomic<br>
> distances and angles, so, be indulgent."<br>
> <br>
> You cannot find in the journal recommendations to authors<br>
> any obligation to force the distances to "be" what you think<br>
> they should be. If we did so, never more we would be able to<br>
> find something different from the range of expected values<br>
> when publishing results based on powder diffraction data.<br>
> <br>
> Of course, when high resolution synchrotron data is available<br>
> for a structure not too complex, the standard of quality may<br>
> be higher than when using laboratory X-rays or neutron data.<br>
> Anyway, they are almost no case known where powder data<br>
> produces better results than single crystal data - when<br>
> available.<br>
> <br>
> On the other hand, letting publish raw refinements, even<br>
> producing clearly wrong interatomic distances, provide<br>
> a job for the experts in redressing bad structures, and you<br>
> get citations ;-).<br>
> <br>
> Maybe we should present 2 tables : one as obtained without<br>
> massaging, the other with....<br>
> <br>
> Comments ?<br>
> <br>
> Best regards,<br>
> <br>
> Armel<br>
><br>
<br>
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