You might like to look at this..
It tries to explain likely twinning possibilities in P21.
If you get C2222 and P21, then probably a~=c - then Beta can have any
value.
C222 axes are then always possible with a* +c* , a*-c*, b* all
having angles ~ 90
Without twinning you wont get 222 symmetry though. Pointless helps here.
Eleanor
http://www.ccp4.ac.uk/dist/html/twinning.html
On 09/27/2011 08:23 PM, Linda Schuldt wrote:
> Dear Yuri,
>
> in a monoclinic space group an orthorhombic lattice metric can be
> simulated when one of the following conditions is fulfilled:
> i) a = c [e.g. in Wittmann& Rudolph (2007) Acta Cryst. D63, 744-749],
> ii) the beta angle is close to 90° [e.g. in Larsen et al. (2002) Acta
> Cryst. D58, 2055-2059 ] or
> iii) c cos beta is about -a/2 [e.g. in Declercq& Evrard, (2002) Acta
> Cryst. D57, 1829-1835]. The a and b axes of the orthorhombic cell are
> identical to the monoclinic a and c axes, respectively. The length of the
> orthorhombic b-axis can also be calculated by "c(monoclinic) cos(beta-90°)
> = 1/2b(orthorhomic)".
>
> I would assume that you have the case iii with a quite high twin fraction.
> If I recall correctly, Declercq and Evrard have a nice figure in their
> paper showing the geometric relationship. If not, let me know and I can
> sent you a figure.
>
> Good luck!
> Linda
>
>
> Yuri Pompeu schrieb:
>> Hello everyone,
>> I have a 2.3A data set that could be scaled in C 2 2 21 and P 1 21 1
>> Intensity statistics tests indicate twinning (pseudo-merohedral h,-k,-h-l
>> in P 1 21 1)
>> I find a good MR solution and when I try to refine it with the twin law I
>> get fairly good maps and decent Rs 21-28%. I can see features tha were not
>> in the search model
>> Which leads me to think that this a valid solution. The one thing that
>> bothers me however is the fact that my beta angle in P 1 21 1 is 104 (not
>> close to 90) and that the geometry gets worse after refinement?
>> Any suggestions?
>> cheers
>>
>
>
>
>
> *******************************
> Dr. Linda Schuldt
> Department of Molecular Biology
> University of Aarhus
> Science Park
> Gustav Wieds Vej 10c
> DK-8000 Århus C
> Denmark
|