[ccp4bb]: p61 vs p65 flipped

[Phil Evans <pre@mrc-lmb.cam.ac.uk>]

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Ed writes:
 > 
 > 
 > Also, can anybody out there tell me whether there are 4 different phasing
 > possibilities: p61 with sites flipped or unflipped, and p65 flipped,
 > unflipped. Or are 2 pairs of these equivalent? I made 4 maps using all 4
 > possibilities and they seem to be different but I'm not sure that I've done
 > everything correctly. For flipped maps I have simply made the Se positions
 > negative in mlphare (i.e. Se site at 0.1 0.1 0.1 becomes -0.1 -0.1 -0.1),
 > and made fft maps for all 4.
 > 


This is a common confusion: there are two solutions in all spacegroups.

The simplest way of thinking about it is consider the whole unit cell,
not just the asymmetric unit

Suppose you have one site/asu, 6 sites/cell. One solution has a site
at x,y,z + all the symmetry relatives in P6(1). The alternative which
also fits the Patterson inverts all 6 sites in the cell to -x,-y,-z.
This inversion automatically changes the 6(1) axis to a 6(5) thus
changing the spacegroup.

Phil