Dear Renu, If the intensity statistics (especially the L-test and Phaser's second moments tests after the tNCS correction) do not indicate twinning, then you probably would not be justified in assuming that the crystal is twinned, unless you find good evidence (e.g. from the symmetry of the MR solution in P1) that the crystal is pseudo-symmetric rather than exactly P21.

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I think if you look closely at the xtriage output, you should find provisos warning you that the results from the tests using potential twin laws can indicate twinning when, in fact, the data are simpy merged in too low symmetry. The tests using potential twin laws are based on the assumption that twin-related reflections should have independent intensities, which is not true *either* when there is twinning *or* when the data have been merged in too low symmetry and the reflections really are related by symmetry. Best wishes, Randy Read > On 2 Feb 2018, at 15:27, Renuka Kadirvelraj <r...@ccrc.uga.edu> wrote: > > Dear All, > > I am trying to refine a 1.9 A structure that indexed with excellent > statistics (XDS) into a primitive monoclinic cell with cell lengths a=81.33, > b=90.11, c=113.25 and beta angle =102.15. Data analysis (Xtriage) indicated > strong pseudo-symmetry with an off-origin peak (53% of origin peak) at > fractional coordinates (0.07, 0.5, 0.19). The intensity statistics did not > indicate twinning and Pointless picked P2(1) as the space group with a 94% > probability. Molecular replacement (Phaser) gave a solution with 4 molecules > in the asymmetric unit with appropriate packing of symmetry-related molecules > and NCS-restrained refinements with TLS led to very good quality maps. > However, the Rfree for the completed model has stalled at 28% (Rwork is at > 24%). > > I could not solve the structure in P2. Data processed in P1 has almost the > same dimensions as the monoclinic cell (a=81.37, b=90.14, c=113.29) and MR > with Phaser located 8 molecules in the asymmetric unit. The data scaled in P1 > also has translational pseudo-symmetry at (0.06, 0.5, 0.185). Intensity > statistics do not indicate twinning but Xtriage detects the two-fold as a > pseudo-merohedral twin (-h,k,-l) andsuggests the likely point group is P2. > Initial refinement indicates the Rfree will likely remain high for the > structure in P1 as well (Rfree ~37% and Rwork at 34%). > > I would really appreciate advice regarding the stalled Rfree and finding the > true space group. > > Many thanks, > Renu > > --------------------------------------------- > Renu Kadirvelraj > Biochemistry and Molecular Biology > 120, Green Street > University of Georgia > Athens, GA 30605 > Tel: (706) 583 0303 > r...@ccrc.uga.edu <mailto:r...@ccrc.uga.edu> ------ Randy J. Read Department of Haematology, University of Cambridge Cambridge Institute for Medical Research Tel: + 44 1223 336500 Wellcome Trust/MRC Building Fax: + 44 1223 336827 Hills Road E-mail: rj...@cam.ac.uk Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk