Dear Junyu,
it looks to me like you encounter a classical monoclinic feature: one
can index monoclinic always in two ways
origin
|
V
A' ---------------------------------- A
\ /\ /
\ / \ /
\ / \ /
\ / \ /
\ / \ /
\ / \ /
\ / \ /
\ / \ /
/________________/
C C'
One cell (A,B,C) has B coming towards you and the other (A',B',C') has
B' pointing away from you. The two axes A and A' have identical length
as have B and B'. But C' is the diagonal in the AC-plane.
In your case you can just swap the A and C axis of the C2 (to follow
the above picture) and then calculate the C' (diagonal) to 136.8.
So to summarize: these are identical cells - just different choice
of axes (and nothing to do with the C2 versus P2 choice ... I think).
Cheers
Clemens
On Thu, May 01, 2008 at 12:03:16PM -0400, Junyu Xiao wrote:
> Hi all,
>
> We run into an interesting space group problem. The same diffraction
> image can be either indexed into space group C2, with a=145, b=44,
> c=67, and beta=110.5; or space group P2 (should be P21 after
> scaling), with a=67, b=44, c=136, and beta=96.8. Both are refined ok
> during index. These two must somehow be related. Can anyone give some
> comments on that?
>
> Thanks a lot,
> Junyu
>
> =================================
> Junyu Xiao
> Department of Biological Chemistry,
> University of Michigan
>
> Lab address:
> 3163 Life Sciences Institute,
> University of Michigan,
> 210 Washtenaw Avenue
> Ann Arbor, MI, 48109-2216
> Phone: 734-615-2078
> ==================================
>
>
>
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