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

Luc Bourhis luc_j_bourhis at mac.com
Fri Dec 8 12:20:18 PST 2006

Hello there,

with the latest cctbx release, the following Python snippet fails

from cctbx.sgtbx import *
info = space_group_info("P2/a")
g = info.group()

The exception indicates that it is not possible to add a centring  
translation with a denominator of 2 to a group which does not have  
any centring translations with that denominator. Of course, in that  
case, that description is trivial because P2/a does not have any  
centring translations!

My real intent was actually to introduce 1/3 centring translations so  
as to deal with a tripled cell. I am just citing the example above to  
show the extent of the problem. My original, failing, snippet was  

from cctbx import sgtbx
s = sgtbx.space_group_symbols("P21/c")
g = sgtbx.space_group(s)

It fails with the same class of exception.

It seems like it is a deeply rooted limitation and that brings me to  
the bigger picture. I am currently working on an EPSRC grant to write  
the next generation of small molecule crystallographic software, a  
joint collaboration between Judith Howard in Durham and David Watkin  
in Oxford (all in UK, just in case…!). The CCTBX is obviously a  
seducing foundation for such a project. Thus we have tried to push it  
into corners which do actually matter to a significant minority of  
small molecule crystallographers.

So I would like to know how difficult it would be to lift the  
limitation at hand. Would it trigger a cascade of changes throughout  
the sgtbx?

Luc Bourhis

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