I think you can find this also from the tables directly.
The origin statement says
Origin at 23
i.e. on *any* special position with PGS 23
there are 4 of them in F23: 4a, 4b, 4c, 4d.
They are listed at the end of the position table and
of course agree with what Ian derived.
BR
-----Original Message-----
From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Ian
Tickle
Sent: Wednesday, August 06, 2008 6:53 AM
To: [log in to unmask]
Subject: Re: [ccp4bb] translation of atom sites and maps
Hi David
No F23 has 4 alternate non-equivalent origins (i.e. invariant amplitudes,
different phases):
(0, 0, 0)
(1/4, 1/4, 1/4)
(1/2, 1/2, 1/2)
(3/4, 3/4, 3/4)
You can of course have other combinations of these by adding any of the
4 F-centring translations.
You can work this out from this table:
http://www.ccp4.ac.uk/html/cheshirecell.html
For F23 (in the list on the right, 2 lines from the bottom) you see that the
Cheshire space group is I-centred (Im3m in the 1st column) and the Cheshire
cell is (a/2, b/2, c/2). The lattice translation gives the origin at (1/2,
1/2, 1/2) and the combination of this with the I centring of the Cheshire SG
gives the origins at (1/4, 1/4, 1/4) & (3/4, 3/4, 3/4).
HTH!
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