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