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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for P21 before giving up and using P1 (you may be able to just re-scale without integrating again, but I'm very out of date); the choice of 'b' axis will be important.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