Re: [sdpd] Better indexing from better data?

[Armel Le Bail <alb...@cristal.org>]

>7. I'll try to keep this offer open indefinitely, but reserve the right to
>limit it if I am overwhelmed with samples.

Overwhelming could occur quickly if all the authors of ICDD
data without spacegroup (some dubious cells there) or
unindexed, were really interested in solving the structures of
their compounds. In the 1995-today range, the total number
of ICDD entries without a space group (or incomplete) is equal
to 1541. The unindexed data subset is difficult to estimate,
maybe ~500, including a not negligible number of Grant-in-Aid
entries. If at least, for these special GiA entries, ICDD had decided
to provide the full powder patterns, the UPPWs could propose
that penultimate test for confirming a probable cell by whole
powder pattern fitting with cell constraint (Pawley or Le Bail
methods - the ultimate confirmation being refining the structure
by the Rietveld method after having completely solved it). But that
decision was not taken (or are these full powder patterns
somewhere ? At least the ICDD entries authors may have kept
them, stored on some diskette since 1995), PDF-2 users have not
even any idea of the FWHM ranges (nobody would try to
index, nor would trust cells from data showing very large FWHMs).

Playing to demonstrate the indexing software efficiency is
different from wanting to realize a complete SDPD and to
publish it. If I would like to solve completely one of these UPPWs,
I would be unhappy if the synchrotron pattern was accessible on the
internet (this could explain why ICDD does not provide the
raw data). But I would not mind to give access if I had failed to
solve the structures (there are 5 unsolved problems with full
powder patterns available inside of the SDPD Round Robin 2 at
                   http://www.cristal.org/sdpdrr2/
and 4 patterns are synchrotron data from ESRF.

The chemists or programmers motivations for realizing a complete
SDPD are different. The programmer can take anything as a
problem to solve for demonstration purpose. The chemist has
to work on restrained subjects and is not necessarily interested
in all unsolved problems around there. Moreover, there is some
deontology more or less applied : if you want solving problems
from other labs, you need to contact them (unless millions
dollars/euros income are predictable ;-).

Best,

Armel
   


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