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

[Luc Bourhis]

Hi Ralf,

>> Now your example would require the matrix of change of basis
>> to be rational whereas in the current implementation it can only be
>> integral.
>
> The change_of_basis_op class has both rational rotation and rational
> translation parts, with a common r_den for the 9 rotation matrix
> elements, and a common t_den for the 3 translation vector elements.
>
> The "r_den" is fixed at 1 only for the space_group class. Since
> space_group_type operates only on space_group objects, the limitation
> applies here too, but the resulting change-of-basis rotation parts
> certainly don't have to be integral (that would be a terrible
> limitation).

I see. Thank you very much for expounding yet another  
misunderstanding of mine. So now I know that I had better putting  
questions to this mailing list instead of trying to figure it out by  
myself from the documentation and the code ;-)

>> At least as far as I understand, from the code and your
>> comments. Thus, the only place where rational numbers would be
>> necessary is in those matrices of change of basis, isn't it? Now,
>> would it be possible to define a new class
>> space_group_irrational_setting which would provide services very
>> similar to space_group_type but accepting non-integral matrices for
>> the change of basis?
>
> You are correct. […]. However, I don't
> think it is worth redoing the entire space_group_type  
> implementation to
> accommodate rational rotation parts.

That was precisely my point.

> […]
> Construct a standard space_group object with the transformed symmetry
> operations and feed it into space_group_type. Multiply the "initial"
> change-of-basis with the output of space_group_type, done.
> […]

Yes, that was exactly what I wished to achieve with that class I  
named space_group_irrational_setting, except that I did not realise  
that space_group_type does already provide the necessary services.

Thank you very much for your patience and thorough answers, Ralf,

Cheers,

Luc