We have data at ~3.2 A for a crystal that looked clearly like primitive orthorhombic while indexing in HKL2000 (or Mosflm). Based on the systematic absences I called it P212121 during scaling. However, we are having some unexpected issues with finding a Mol. Rep. solution using a model that is present as is in the crystal (along with regions that are absent in the model). There seems to be a clear non-origin Patterson peak at ~0.5, ~0.5, ~0.5. In agreement with this, when I use Molrep to find two molecules of the model in the asymmetric unit, the two solutions are derived from the same rotation function but with translation solutions displaced exactly 0.5, 0.5, 0.5 fractional coordinates from each other. However, the maps after refinement don't look impressive at all.
I looked at h+k+l = 2n+1 reflections. Visual inspection suggests that several of these are weaker than the corresponding h+k+l = 2n reflections, but there are certainly exceptions. I have not done a systematic calculation of what the average I and sigma are for h+k+l = 2n+1 and h+k+l = 2n are. Does this smell like a wrong space group assignment? Twinning? Or am I missing something very trivial here? Or is this a pseudo-translation we have to live with? Any kind of advice will be very helpful. I am happy to provide more information if you tell me what you need. Thanks a ton in advance. Best, JK P.S. Matthew's coefficient indicates that there could easily exist up to 6 monomers in the asymmetric unit (4 monomers if 70% of the crystal is solvent).
