[sdpd] Re: Another 2 pence worth

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

Kenneth wrote :

> As regards the space group determination problem,
> I can recommend watching out for the Jan 2001 issue
> of Acta A, which will contain a paper from RAL by
> Anders Markvardsen on that very topic.  It outlines
> a probabalistic approach to SG detn from powder data
> that puts SG detn on a fully quantitative basis.

Bayesian salt I suppose ?

We have seen in the last 10 years the Bayes's theorem
applied to almost all topics in crystallography. And
in all cases, well, this is always better with Bayes
than without Bayes...

There are comments about the Bayes's theorem in the
Encyclopedia of Physics (VCH, 2nd Edition, 1991, p.962)
that seems to put some doubt about it. I cite just
a part of the comments :
"The difficulty of the theorem lies in interpreting what
is meant by the prior and posterior "probabilities"
of a hypothesis, though if these can be somehow
surmised, the theorem tells us (or should tell us)
how experiment ought to influence our attitude to
a hypothesis. Presumably (though this presumption
has been a battleground among scientists and
philosophers for over 200 years) we should prefer
the hypothesis with the greatest posterior probability".
etc.  

My approach of SG determination is more pragmatic
than probabilistic : if the space group is ambiguous
then I test all apparently possible space groups.
Ambiguity may come naturally (space groups with same
extinctions) or due to overlapping. Not that complex.
I would not give the decision to any probabilistic
software. I prefer to decide myself.

Best,

Armel Le Bail
http://www.cristal.org/course/



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