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
