[sdpd] DIRDIF applied to SDPDRR sample 1

[Armel Le Bail <armel... @fluo.univ-lemans.fr>]

Dear Members,

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This mail to say that not only the pharmaceutical sample
2 of the SDPD Round Robin could have been solved by
DIRDIF, by using the ORIENT option that locate a known
fragment. 

For sample 1 also, it was possible easily to guess the
existence of at least a [CoN5O] octahedron from the given
formula [Co(NH3)5CO3]NO3.H2O. Building such an
octahedron inside the ORUSER base of fragments is
quite easy. I have verified that DIRDIF was able to locate
this octahedron, regardless of the O position, from either
the full dataset of structure factors as extracted by FULLPROF
or any reduced dataset built by excluding reflections having
a neighbouring one at less that 0.05 2-theta degrees. Those
data are available at :
       http://www.cristal.org/SDPDRR/sample1.html

Supplementary atoms are proposed by the DIRDIF Fourier
synthese, so that an uncomplete solution is built that provides a
Rietveld RB factor as low as 20%, sufficient for refining
and going to the complete solution by further Fourier
difference syntheses. 

Of course, a CO3 group attached to this octahedron with
the possibility to vary their relative orientations would have
been better. And also, adding a second fragment like NO3
plus a water molecule would have even completed the
structure. Unfortunately, this is not yet possible with the
DIRDIF software, accepting only one fragment.
Anyway, DIRDIF worked well on this case !

Still no more sophisticated software for molecule location
available on the market, apart Cerius2 ???

Best regards,


Armel Le Bail - Universite du Maine, Laboratoire des Fluorures,
CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans Cedex 9, France
http://www.cristal.org/