I should expand a bit: on this BB last year Ian Tickle pointed out
that the IUCr "standard" for primitive orthorhombic space groups is to
have a <= b <= c, irrespective of which axes are 2s or 2(1)s. Thus eg
space group 18 in its "standard" setting may be P 2 21 21, P 21 2 21
or P21 21 2: the last of these is the "reference" setting.
Similarly , space group C2 should be reindexed as I2 if this gives a
smaller beta angle. Ralf Grosse-Kunstleve has recently pointed this
out to me as well.
I have changed Pointless (from version 1.2.12) to follow this
convention, though it is a user-settable option to use the reference
setting. It does seem sensible to follow the standard convention.
If you are picking up the space group from an MTZ file, it is
unambiguous, similarly from the space group name in a PDB file. I
think Arp/warp should allow whatever setting is in the input file.
Phil
On 10 Jun 2008, at 16:44, Victor Lamzin wrote:
Dear Phil,
One reason has been simplicity - many ARP/wARP modules operate with
space group number only. For space group 18 this would mean P21212.
Using space group name might be less robust - I remember some
compatibility problems when CCP4 introduced spaces into space group
names, this broke some of the parsers, including simple-minded ones
from ARP/wARP.
If there is, however, a strong wish for ARP/wARP to support
'unconventionally' indexed space groups - then we will certainly try
to introduce it.
With best regards,
Victor
PhilEvans wrote:
Is there a reason why Arp/warp doesn't like space group P 21 2 21?
Phil
