Se the twinning rules in http://www.ccp4.ac.uk/dist/html/twinning.html
P3
pace group number space group point group possible twin operators
143 P3 PG3 -h,-k,l; k,h,-l; -k,-h,-l
144 P31 PG3 -h,-k,l; k,h,-l; -k,-h,-l
145 P32 PG3 -h,-k,l; k,h,-l; -k,-h,-l
146 H3 PG3 k,h,-l
ie you need to test -h,-k,l (TWIN -1 0 0 0 -1 0 0 0 1)
k,h,-l )TWIN 0 1 0 1 0 0 0 0 -1)
-k,-h,-l (TWIN 0 -1 0 -1 0 0 0 0 -1)
Eleanor
pointless or sfcheck will suggest which is the most likely to be real.
Eleanor
Kristof Van Hecke wrote:
Dear,
Sorry for the off-topic question.
I'm facing a (probably) merohedral twinning problem, regarding a small
molecule.
Using Xprep, I get a Hexagonal P-lattice with cell:
18.014 18.014 22.048 90.00 90.00 120.00
Mean |E*E-1| = 0.902 [expected .968 centrosym and .736 non-centrosym]
However, based on the systematic absence exceptions, the probable
(apparent) SG's are:
P6(3)/m (Laue '6/m')
P6(3) (Laue '6')
P6(3)22 (Laue '622')
61/65 62=31 63 -c- --c
N 60 50 36 2471 1420
N I>3s 19 19 0 420 161
<I> 186.6 223.1 4.6 30.0 15.5
<I/s> 2.3 2.6 0.3 1.6 1.2
I know there is a twin law to transform the (apparent) Laue group
'6/m' to the (true) Laue group '-3'
(TWIN law -1 0 0 0 -1 0 0 0 1) and merging the data in a trigonal SG,
but this is not solving the structure at all...
Has anyone noticed a similar case that could be of any help please..?
Many thanks
Kristof
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