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Hi,
Thank you for your very useful information.
By using the program of Degree of Pseudosymmetry Estimation on Bilbao
crystallographic server, I found the reported data with space group
I-4c2 has degree of pseudosymmetry about 0.84 to supergroup (I4/mcm).
What does this mean? Where I can find the a formal definition of
Degree of Pseudosymmetry?
Thanks!
Best Regards!
Bin Li
At 11:46 PM 8/2/2006, you wrote:
The Bilbao crystallographic server might be of some use.
(http://www.cryst.ehu.es/)
Check for instance the Wykoff splitting when going to a lower symmetry:
I4/mcm (140) -> I-4c2 (120)
wykoff splitting table:
I4/mcm I-4c2
32m -> 16i 16i
16l -> 16i
16k -> 16i
16j -> 16i
16i -> 8e 8e
8h -> 8h
8g -> 8g
8f -> 8f
8e -> 8e
4d -> 4d
4c -> 4b
4b -> 4c
4a -> 4a
(Looking at this table, I do not understand the splitting in positions
you observe, unless I am missing a transformation somewhere ....)
The coset decomposition between the two spacegroup gives a clue of
which symmetry element is missing between the two spacegroups:
A representative elements of the left cosets of I4/mcm and I-4c2 are
1. x,y,z (of course)
2. (-y,x,z) (a 4 fold along z)
HTH
Peter
----- Original Message -----
From: Bin Li <[EMAIL PROTECTED]>
Date: Wednesday, August 2, 2006 8:48 pm
Subject: RE: [ccp4bb]: Question about symmetry relationship
> *** For details on how to be removed from this list visit the ***
> *** CCP4 home page http://www.ccp4.ac.uk ***
>
>
> Dear BR:
>
> Thank you for your quick reply.
> From my data and my experience I believe my refinement with high
> symmetry space group is right.
> The problem is that if I assume both structures are right, how I
> can
> describe the atom site relationship in both structures in terms of
> symmetry, such as removing inversion center , or mirror planes.
>
> By the way, if I transfer my high symmetry space group (I4/mcm) to
> lower symmetry space group (I-4c2, which is also a subgroup of
> I4/mcm), I got the atom sites of A, B, and D at 4a, 16i, and 8h
> sites, respectively, in stead of the one reported in the
> literature
> (4d, 16i and 8e sites). Is this a problem or not?
> I hope I am not confusing you.
> Your help is highly appreciated!
>
> Best Regards!
>
> Bin Li
>
>
> At 10:24 PM 8/2/2006, you wrote:
> >The first thing I would look at is the |E**2-1| number
> >in the intensity distributions.
> >This should indicate whether centrosymmetric or not.
> >0.736 for acentric and 0.968 for centric structures.
> >
> >If this is not decisive in small molecules, suspicion
> >is in order, but w/o more stats it is hard to say.
> >
> >BR
> >
> >
> >-----Original Message-----
> >From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On
> Behalf Of Bin
> >Li
> >Sent: Wednesday, August 02, 2006 7:47 PM
> >To: [email protected]
> >Subject: [ccp4bb]: Question about symmetry relationship
> >
> >*** For details on how to be removed from this list visit the ***
> >*** CCP4 home page http://www.ccp4.ac.uk ***
> >
> >
> >
> >
> >Dear:
> >I have a question about symmetry relationship between two space
> groups.>In the literature, there is a compound AB4C2. It was
> refined with
> >I-4c2 space group and A, B, and C are at 4d, 16i and 8e sites,
> respectively.>I got a new compound AB4D2 which can be refined very
> well with high symmetry
> >space group, I4/mcm. In my compound A, B, and D are 4a, 16l, and
> 8h sites,
> >respectively. If I draw both structures, they look same, except
> some bond
> >distances change.
> >The high symmetry space group is central symmetry and the another
> one in
> >non-central symmetry, but both have very similar Wyck positions.
> >How do I figure out the symmetry relationship between these two
> structures?>Your help is highly appreciated!
> >Thank you for your time!
> >
> >Best Regards!
> >
> >Bin Li
> >
> >Bin Li, Ph.D
> >Ames Lab., Iowa State University
> >Ames, IA, USA
>
