[sdpd] Re: Scratch

[L.M.D.Cranswick... @dl.ac.uk (L. Cranswick)]

The problems of impurities in powder samples and indexing -
and the possibilty of reliable ("black box") powder indexing 
method might be realisable Re: the recent work by the following: 
Benson M. Kariuki, Scott A. Belmonte, Malcolm I. McMahon, 
Roy L. Johnson, Kenneth D. M. Harris and Richard J. Nelmes in 
the recent paper in J. Synchrotron Radiation, (1999), 6, pp 87-92.
"A new approach for indexing powder diffraction data 
based on whole-profile fitting and global optimization using a 
genetic alforithm".

This combines Le Bail whole pattern fitting with Genetic
Algorithms for the optimisation.  Hopefully there may be
an available software package to show this in action at 
the Glasgow IUCr Software Fayre
    http://www.ccp14.ac.uk/projects/iucr99-softwarefayre/

In theory, overlapping peaks (not having wonderful
synchrotron data) might also not be that 
big a problem; again, due to optimising on the whole
profile.

If this is a new robust method for powder indexing, it 
would be a shame to see it just gathering dust in the 
printed literture.   One perceived problem with new algorithms
for structure solution from powder diffraction data is that 
many of them don't seem to make it beyond the printed word.

Though in the case of indexing, it sounds like the majority
of users must be happy with the combination of Ito, Treor,
Dicvol - otherwise - they would be taking the ideas in
the above and similar journal articles and converting it
to executable code post haste? (i.e., using Le Bail fitting
combined with an appropriate optimisation algorithm - or
suite of algorithms - of which the genetic algorithm seems
to be one of the more obvious ones to use as described in
the above reference)

---

As an extra aside - given the availability of Le Bail fitting
in most Rietveld software - optimising to give the best fit
for indexing might also give an option for automatic suggestion 
of space groups.

Lachlan.

> Of course, proper indexation is the condition sine qua non.
> Ito, Dicvol and Treor are already extremely reliable. Using 
> all of them ensure you evenmore success. The problem is
> that they work wonderfully only with wonderful data. I have
> heard about many other software and even recent use of
> Monte Carlo for indexing. That those software are more
> comfortable with bad starting data than Ito, Dicvol and Treor,
> I do not believe it. Then, the state of the art, in my opinion, 
> continue to be the simultaneous use of Ito, Dicvol and Treor.
> The biggest problem on which you have little influence is 
> your sample purity. Not being sure of the purity is sufficient
> for discouraging many researchers to adopt powder
> diffraction ;-).
> 
> Armel
> 
> Original mail :
> From: Gregory A Stephenson <STEPHENSON_GREGORY_A... @LILLY.COM> 
> 
> >Although molecular finding methods may be considered a not so useful technique
> >since "the structure" is already known from NMR or mass spectrometry (or a
> >chemist who actually has a reasonable idea of a few possibilities of what he
> >has synthesized-which is fortunately generally the case).  It still appears to
> >me that in the event that you can rely on the latter, you still need the basic
> >starting point-proper indexation.  I use the standard Ito, Dicvol and Treor
> >programs; each being very good-but indexation still seams to be more limiting
> >than the monte carlo/simulated annealing approach to solution.  What is the
> >state of the art?  Is there something more reliable than these programs for
> >low symmetry organic molecules?
> 
> 
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-- 
Lachlan M. D. Cranswick

Collaborative Computational Project No 14 (CCP14)
    for Single Crystal and Powder Diffraction
Daresbury Laboratory, Warrington, WA4 4AD U.K
Tel: +44-1925-603703  Fax: +44-1925-603124
E-mail: l.cranswick... @dl.ac.uk  Ext: 3703  Room C14
NEW CCP14 Web Domain (Under heavy construction):
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