[cctbxbb] Fail to extend space-group with centring translations

David Watkin david.watkin at chemistry.oxford.ac.uk
Mon Dec 11 00:26:19 PST 2006

	Since none of you guys have come across the need for dreamed-up space groups, I 
guess the need is somewhat limited.  These artifacts are tools used to help in 
the understanding of crystal (not molecular) structures so rarely get published.

 > Could you please outline a plausible example where the current
 > implementation is a serious limitation?

I believe Ralph can handle C-1 so the attached example would not be a problem, 
but it demonstrates the situation.
Table 3 gives basic parameters for four compounds. It is evident that I and II 
are related, and that III anf IV are related.  What is not evident without 
computation is that all four CRYSTAL STRUCTURES are related.  This becomes 
evident if you transform I and II to the correct C-centred cell.

In each pair of tables 4 & 5 and 6 & 7 atomic parameters can be compared 
directly. If the P to C transformation is made, all four tables can be compared 
by inspection.

In this case, the referees and editors refused to accept a non standard SG, so 
the text of the paper becomes laborious, explaining what would otherwise have 
been obvious.

In higer symmetry SGs it may be easier to compare structures if even more 
bizzare SG are invented, or if non-standard origins are used.

> You can create any P1 cell you want, it is totally trivial: first use
> e.g. xray_structure.expand_to_p1(), then loop over the translations you
> want to duplicate the atoms. We can easily add methods to do it in one
> call. Note that the cctbx is open source, with the possibility of write
> access to the main source repository.

Im sure this will be true, and that someone will provide the user-interface for 
generating the basis vectors form the new (probably enlarged) cell and new 
atomic parameters after the expansion.

Im sure the SM-tbx folk have more pressing problems to deal with than worrying 
about this minority application

Best wishes
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