Twin laws are possible if there are 2 ways to index your cell, and non-merefedral twinning is possible in any system depending on the cell.
I am not sure of the small molecule tools to check twinning though. Eleanor Dodson On 13 August 2014 05:58, Lijun Liu <lijunli...@gmail.com> wrote: > Hi, Pc may not be the space group for your crystal, if the molecule is > chiral. Seems like the data were forced to be reduced to a mirror_related > SG. Lijun > On Aug 13, 2014 2:00 AM, "Kristof Van Hecke" <kristofrg.vanhe...@gmail.com> > wrote: > >> Dear, >> >> I’m struggling with the following (small molecule) problem: >> >> We are trying to solve the structure of a metal-organic framework >> containing a chiral compound. >> The space group is most probably Pc, but when refining, SHELX gives the >> error “Possible racemic twin or wrong absolute structure - try TWIN >> refinement”. >> As we know our compound is enantiopure, a racemic twin is very unlikely. >> In this regard, also a centro-symmetric space group is not possible >> (although CrysAlisPro always gives P2/c as the proper space group). As a >> matter of fact, trying different space groups is not solving the problem. >> >> The second problem is that half of the structure is visible, but the >> other half is completely not clear. Refinement is not possible at all >> (R-value of 33%). >> When running TwinRotMat (Platon), I get the following possible 2-fold >> twin axes: >> >> 2-axis ( 0 1 -1 ) [ -2 5 -4 ], Angle () [] = 2.31 Deg, Freq = >> 14 >> ************* >> (-0.992 -0.019 0.015) (h1) (h2) Nr Overlap = >> 84 >> (-0.430 0.075 -0.860) * (k1) = (k2) BASF = >> 0.96 >> ( 0.459 -1.146 -0.083) (l1) (l2) DEL-R >> =-0.064 >> >> 2-axis ( 0 1 1 ) [ 2 5 4 ], Angle () [] = 2.31 Deg, Freq = >> 15 >> ************* >> (-0.992 0.019 0.015) (h1) (h2) Nr Overlap = >> 229 >> ( 0.430 0.075 0.860) * (k1) = (k2) BASF = >> 0.94 >> ( 0.459 1.146 -0.083) (l1) (l2) DEL-R >> =-0.050 >> >> 2-axis ( 1 0 -2 ) [ 3 0 -2 ], Angle () [] = 0.54 Deg, Freq = >> 19 >> ************* >> (-0.136 0.000 -0.576) (h1) (h2) Nr Overlap = >> 992 >> ( 0.000 -1.000 0.000) * (k1) = (k2) BASF = >> 0.86 >> (-1.703 0.000 0.136) (l1) (l2) DEL-R >> =-0.030 >> >> 2-axis ( 1 2 -1 ) [ 5 5 -1 ], Angle () [] = 0.39 Deg, Freq = >> 13 >> ************* >> (-0.380 0.620 -0.124) (h1) (h2) Nr Overlap = >> 854 >> ( 1.256 0.256 -0.251) * (k1) = (k2) BASF = >> 0.88 >> (-0.617 -0.617 -0.877) (l1) (l2) DEL-R >> =-0.019 >> >> However, none of these do actually improve the refinement. >> >> >> Has anyone encountered possible twinning/twin laws in Pc please? >> Or any other suggestions are most welcome? >> >> >> Thank you very much >> >> Kristof >> >
