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On Wed, Jul 9, 2014 at 5:14 PM, Chris Fage <[log in to unmask]> wrote: > Despite modelling completely into great electron density, Rwork/Rfree > stalled at ~38%/44% during refinement of my 2.0-angstrom structure > (P212121, 4 monomers per asymmetric unit). Xtriage suggested twinning, > with <|L|> = 0.419, <L^2> = 0.245, and twin fraction = 0.415-0.447. > However, there are no twin laws in this space group. I reprocessed the > dataset in P21 (8 monomers/AU), which did not alter Rwork/Rfree, and > in P1 (16 monomers/AU), which dropped Rwork/Rfree to ~27%/32%. Xtriage > reported the pseudo-merohedral twin laws below. > ... > Performing intensity-based twin refinement in Refmac5 dropped > Rwork/Rfree to ~27%/34% (P21) and ~18%/22% (P1). Would it be > appropriate to continue with twin refinement in space group P1? It sounds like you have pseudo-symmetry and over-merged your data in P212121. I would try different indexing for P21 before giving up and using P1 (you may be able to just re-scale without integrating again, but I'm very out of date); the choice of 'b' axis will be important. If none of the alternatives work P1 may be it, but I'm curious whether the intensity statistics still indicate twinning for P1. -Nat