[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()
g.expand_ltr(tr_vec((1,1,1),2))
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
therefore
from cctbx import sgtbx
s = sgtbx.space_group_symbols("P21/c")
g = sgtbx.space_group(s)
g.expand_ltr(sgtbx.tr_vec((1,0,1),3))
g.expand_ltr(sgtbx.tr_vec((2,0,2),3))
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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