[Cod-bugs] spacegroup name 'C 2 e b'

[Marcin Wojdyr]

Hi Saulius,

> I the meantime, may I suggest that you either use the Hall symbol ('C
> -2ac 2'), which seems to be correct for all 9 COD entries that mention
> 'C 2 e b' [1]. I have even a code (in Ada :) that parses the Hall symbol
> and returns symmetry operation matrices [2]. Alternatively, probably the
> most versatile way to describe symmetry is to use symmetry operation
> matrices described in the '_space_group_symop_operation_xyz' data item.
> All entries in [1] seem to have these data items.

That's actually why I was checking the space groups. I was trying to
find out what's the best order for reading a space group. On one hand,
Hall symbols are unambiguous, but on the other hand if they are less
popular, there is a bigger chance that someone changes H-M in a file
and forgets to change Hall. At least that's my thinking.

I also have code for parsing Hall symbols, although I'm missing
software for doing the opposite (symops -> Hall symbol). In
particular, for getting canonical Hall symbols. ITC vol. B mentions an
unambiguous method of selecting the Hall symbols by sorting operations
into "a strictly prescribed order based on the shape of their Seitz
matrices", and then it refers for details to a website that's no
longer available. Cctbx docs mention that they used Xtal 3.7 to
generate Hall symbols, but I haven't sorted out how to use Xtal.

Best,
Marcin

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