[sdpd] Re: UPPW-5 solution - UPPW-6 problem

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

Hi,

Thanks Alan.

Successful indexing depends on so many parameters :
compound crystallinity, cell complexity, data quality, software
capability, human expertise. Sure, the best is to have an excellently
crystallized compound (in fact a single crystal is better ;-), to have
a powder pattern recorded on a 3rd generation synchrotron, no impurity,
and to apply several "exhaustive" programs : the chances of success
for a medium-sized structure will be close to 99.99% if the job
is done by a well-trained scientist.

But life is not always so kind. Samples do not want to crystallize,
budgets do not allow for affording the best in-lab diffractometer,
the nation budget is bot large enough for offering you your easily
accessible synchrotron facility, Commissions examine your
proposal and refuse it sometimes, etc.

The fact that these "Grant-in-Aid" ICDD patterns stay unindexed
seems to indicate that the authors did not believe to the cells they
obtained from their indexing program (or they did not even try ??).
Up to now, the UPPWs show that at least those of these recent
unindexed patterns  that were recorded correctly and carefully
have a good chance to be indexed without real difficulty. Here at
the SDPD mailing list, we try to promote SDPD, right ? The purpose
is to show that indexing is more easy than some are believing (though
it requires very careful work).

>UPPW-5 is a case where powder data does not yield a unique solution.
>When multiple solutions yield similar "perfect" Pawley/Le Bail fits
>with similar de Wolff  values then it is not a matter of failure of
>the programs/methods but rather a failure of the data to yield a
>unique solution. In my view it is therefore not possible for any
>indexing method to resolve the ambiguity.

Of course, some of the UPPWs were 'easy' and the most
probable cells proposed have large chances to be the correct
ones. For UPPW-5, the most probable cells may be uncorrect.
In all cases we will have to wait for the structure solutions
before to claim any success.

About "exhaustivity", it does not mean "successful in all cases".
It only means "considering all aspects of the problem" - eventually
before failing ;-). In my opinion, not considering the possibility for
an impurity presence would yet remove that "exhaustivity" label.
Anyway, well-selected words are used for markettng and packing
Science. Scientists are particularly careful with their publication
titles... It is similar to the promotion of a new perfume.

>Now having said that no method is going to resolve
>ambiguity, it is my opinion that a combined ITO and DICVOL probably
>solves more than 90% of every thing thrown at it. Thus new algorithms
>are only filling a small gap and to find this gap is presumably what
>UPPW is all about - or is it? If not then it's a lot of fun in any
>case.

Yes, UPPWs will serve as materials for discussing recent indexing
progress (if any) in a review paper.

During the last five years were made efforts as large as during
the previous 30 years for solving only 10% of the most difficult
problems. This could be a general consideration in Science  - making
progress is more and more difficult unless some decisive advances
are made. But as long as samples will refuse to crystallize, and
budgets will refuse to grow enormously, a major breakthrough
in indexing would have no impact at all... fortunately - so that
Powder Diffractionnists will keep their job for a while ;-).

Let us continue to have fun.

Best

Armel

PS - If you wish some simulated pattern with known solution,
this could be proposed also as one of the next UPPWs.
Using some newly predicted structure, for instance... - even not yet
synthesized ;-).


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