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

[David Watkin]

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.
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.

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.

Best wishes
DAvid

Luc Bourhis wrote:
> Hi Ralf,
> 
>> Hi Luc,
>>
>> You can make both examples work like this:
>>
>> […]
> 
> Thanks for your explanations. I was overly pessimistic then and good 
> news anyway! I did not understand the logic of the code at all in fact…
> 
> Luc
> 
>