Dear colleagues,
Since the restrained SDPD future has been announced in the respective list
without a chance to go back:
http://tech.groups.yahoo.com/group/sdpd/message/3016
It's my turn to put forward an alternative unrestrained variant that seems a
bit more interesting and not so hopeless, IMHO.
This variant consists in a more accurate approach to the experiment and the
modeling instead of compensating the inaccuracy by restraints.
In the discussion of the restrained Rietveld:
http://dx.doi.org/10.1107/S1600536813017522
proposed as an instance of the SDPD future trend, I presented an unrestrained
variant of the refinement:
http://tech.groups.yahoo.com/group/sdpd/message/3023
and noted that Future improvements are possible if one exerts further efforts
to the sample preparation, measurements and the refinement model adjustment.
Spending an extra couple of hours for the data analysis, an additional
micro-admixture has been identified whose inclusion into the model resulted in
further notable improvements of the structure geometry:
http://sites.google.com/site/ddmsuite/home/vn2075-DDM.cif
http://sites.google.com/site/ddmsuite/home/vn2075-DDM.png
Characteristic distances and their expectations:
Pos. Dist. Exp.
Co-O1 1.92(2) 1.91
Co-O2 1.94(2) 1.91
Co-N1 1.98(1) 1.96
Co-N2 1.95(2) 1.96
Co-N3 1.92(2) 1.96
Co-N4 1.99(1) 1.96
C1-O1 1.31(4) 1.32
C1-O2 1.34(4) 1.32
C1-O3 1.25(3) 1.23
N5-O4 1.27(3) 1.24
N5-O5 1.23(3) 1.24
N5-O6 1.25(3) 1.24
All distances and intramolecular angles are in agreement with the expected
values within their uncertainties. Future improvements are still possible,
being determined mostly by the sample preparation and measurements now.
Of course, in REALLY complex and HOPELESS cases a minimal number of constraints
may be imposed if the refinement convergence of atomic positions is REALLY
impossible without them. In the above example, rigid constraints were imposed
on H-atoms, which is, though, not always necessary.
The use of soft restraints should be avoided as much as possible since a
generally acceptable methodology of their safe application has not been
developed yet.
Best unrestrained regards,
Leonid
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Leonid A. Solovyov
Institute of Chemistry and Chemical Technology
660049, K. Marx 42, Krasnoyarsk, Russia
http://sites.google.com/site/solovyovleonid
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