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

Ralf W. Grosse-Kunstleve rwgk at yahoo.com
Sun Dec 10 21:19:54 PST 2006

> Luc, This is a key item in Ralphs reply:
> "You cannot change the rotation denominator (fixed at 1) for the
> matrices processed by sgtbx.space_group. Therefore you cannot work
> with, e.g. the pseudo-orthorhombic C-centered setting of a hexagonal
> space group. However, I never came across a situation where this
> was interesting."
> If the design of the toolbox is too restrictive to permit the user to
> invent any closed set of operators they want - well thats the end of it.
> As we say in the UK, C'est la vie.

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

> There were lots of things we would have liked to add to CRYSTALS, but
> there was no room in Bob Carruthers database, and extending it would
> have been a major hasle.

I am sure the cctbx is more flexible.

> As you pointed out, there are work-arounds that can be applied by a
> clever crystallographer, and the less clever ones won't even try to do
> anything clever.  It's a self-limiting situation!  Much of what crystal
> enginners want could be done if Oleg lets the user create an extended
> atom parameter list, and put this into a new unit cell in P1.

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.


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