Matheus
A couple of points
Firstly, isn't this pseudo merohedral rather than merohedral. I realise you
didn't mention either in your email but it is in the title of the previous
message.
Secondly, the difference in the cell dimensions (b=123.92 and c=128.89A)
appears to be quite large and should lead to split spots which (I think)
corresponds to non merohedral twinning. Did you observe these but integrated
them as one?
My main interest in this is whether, with a high resolution set up (low
divergence beam and high resolution detector), the individual contributions
could be separated out. Secondly whether it would be useful to do so e.g. for
the case where the twin fraction is near 50%
Regards
Colin
(Regarding what to call the twinning I have some sympathies with Humpty
Dumpty's view "When I use a word... it means just what I choose it to
mean-neither more nor less" As important a philosopher as Wittgenstein.)
________________________________
From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of
Matheus Pinheiro
Sent: 22 June 2010 12:00
To: CCP4BB@JISCMAIL.AC.UK
Subject: Re: [ccp4bb] Merohedral twining for P212121.
Hi Yang,
I had the experience to work with twinning in P212121 space group. My
crystals belong to space group P212121 with unit cell dimensions a=82.39,
b=123.92 and c=128.89A. You can see that "b" and "c" are not equal but very
simmilar, and this fact can allow twinning for this space group. The crystals
with higher fraction of twinning can be easier processed in tetragonal space
group P41212 with unit cell dimensions a=b=126.95 and c=82.43A. However the
packing is actually orthorhombic. In my work, to fully assess the twin fraction
and the twin operator was employed the program PHENIX (xtriage tool). I solved
my crystal structure using data suggested by PHENIX program (twin operator
(-h,-l,-k)).
Good luck!
Matheus
On Mon, 21 Jun 2010 13:52:05 -0700, yang li wrote
> Dear all,
>
> Is there any possible of twining for a normal P212121
spacegroup and what is the twin law, say no equal cell dimensions?
> Some people said there is no twin law for such symmetry but I am not
very sure. Thanks a lot.
>
> Best
> Yang
-------------------------------------------------------------
Matheus Pinto Pinheiro
Laboratório de Cristalografia de Proteínas
Departamento de Física e Química
Faculdade de Ciências Farmacêuticas de Ribeirão Preto - USP
Brasil
Fone: +55.16.3602.4193
m...@fcfrp.usp.br
