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[sdpd] Restrained SDPD future
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<p><br>
>Rapid-fire conclusions and emotions instead of careful analysis again...<br>
<br>
For the paper acceptation, I had first (among other queries) to convince the<br>
referee that I could really extract 20 peak positions from the <br>
pattern for indexing.<br>
It is true that it is difficult to obtain more than 20 such good peak <br>
positions due<br>
to the presence of the hemihydrate. The evidence is there :<br>
<a href="http://www.cristal.org/sdpd/co.jpg";>http://www.cristal.org/sdpd/co.jpg</a><br>
Convincing ? Would have you even tried to index ?<br>
<br>
Indeed, this case is exactly the example of what should not be done :<br>
- 20% impurity : so prepare another sample...<br>
- large preferred orientation : make a new pattern free of it...<br>
- laboratory diffractometer : ask for synchrotron time...<br>
- etc (+ careless analysis : be more careful ;-))<br>
Anyway, the data were sleeping unsolved in my drawers since 1998 and I<br>
have currently no time to waste in improving them (why, since the SDPD was<br>
successfull finally - I am emptying my drawers before retirement). And see<br>
what you can do with those ~20 peaks (once I provided you with the <br>
solution...).<br>
<br>
About the Alan comment at the Rietveld mailing list :<br>
>It is truly a "tour de force" to quote N-H bonds determined to 3<br>
>significant figures from a heavy atom compound with one axis as long<br>
>as 17.5 Angstroms - by x-ray powder diffraction !<br>
<br>
That "tour de force" is due to the Rietveld software, to which it is said to<br>
restrain the N-H distances to 0.96(1), so it is then just logical that the 12<br>
N-H distances are all between 0.946(10) and 0.969(10). If there is a "tour<br>
de force" here, it was to index the powder data, and then to obtain a<br>
starting structure model for the refinement, thanks.<br>
<br>
About ESDs and the Rietveld method, the "Rietveld refinement guidelines"<br>
refer to old papers which established more or less that it would be better<br>
to multiply the Rietveld ESDs by a factor 3 at least, since they were so<br>
largely underestimated (you know, overlapping...). If all experts recognized<br>
this fact, no Rietveld software programmer changed his ESDs output. So,<br>
even with DDM, when Leonid proposes seriously P-O = 1.72(3) for K4P2O7<br>
as being an "information", I see 1.72(9), and this is not an information of<br>
interest to me (can be included in the book of most horrible P-O distances<br>
ever obtained) - hence the need for restraints ;-) (and, in this case, the data<br>
were from ID31, ESRF, hard to improve anything).<br>
<br>
The future of SDPD is in restraints, restraints and more restraints, sadly.<br>
<br>
Or in DFT ?<br>
<br>
But DFT is only possible once the indexing and structure solution<br>
"tours de force" are done. In all my own cases where a DFT was realized<br>
in order to improve the poor Rietveld refinement results, the DFT started<br>
from the poor Rietveld refinement results - generally with restraints ;-).<br>
<br>
Best,<br>
Armel<br>
<br>
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