Hi Luc,
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))
If you do it like this: from cctbx import sgtbx info = sgtbx.space_group_info("P2/a") g = info.group() t = sgtbx.tr_vec( (1,1,1), 2 ) t = t.new_denominator( g.t_den() ) g.expand_ltr( t ) print sgtbx.space_group_info(group=g) The trick lies in obtaining the translational denominator from the group it self and setting that value in your t vector. The same issue is there when multiplying in an extra rotation operator. In that case you need the g.r_den().
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))
from cctbx import sgtbx info = sgtbx.space_group_info("P2/a") g = info.group() g.expand_ltr(sgtbx.tr_vec((1,0,1),3).new_denominator( g.t_den() ) ) g.expand_ltr(sgtbx.tr_vec((2,0,2),3).new_denominator( g.t_den() ) ) print sgtbx.space_group_info( group=g ) works.
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?
The r_den, t_den thing is confusing and using an approach with rational numbers would make sense. time time ... Cheers Peter