[phenixbb] problem with high Rfree

[Christina Bourne]

Hello BB,
I am working on a structure that is twinned (merohedral) in P41.  Initial estimates of twinning fraction (via xtriage and Yeates server) are around 0.37; in refinement they start around 0.49 and drop to 0.46 (so far).  My MR soln is good (Phaser LLG ~550) with 4 mols per AU; this gives good packing with no room for another mol.  I have data from 48-2.8A.
I should mention that data processing wasn't easy- this crystal form also has an apparent epitaxial twin in the b dimension causing a doubling in that dimension.  During indexing (with SAINT) I applied a transformation of 100 00.50 001 to the reflections, which then gave me the expected cell and from Xprep the data looks OK.  (Yes, I am trying to grow different forms)
Maps look nice, although I wish my ligand had more/better density.
I ran phenix.refine with defaults, and included twin_law="h,-k,-l", xray_data.r_free_flags.generate=true, detwin.map_types.aniso_correct=true and also simulated_annealing=true.  My starting R/ Rfree is 0.448/ 0.4385; ending is 0.2962/ 0.4270.  Geometry is somewhat tighter after refinement but is better (bonds 0.017 to 0.01, angles 1.966 to 1.530).  I have not included my ligand, and I can see a few spots where the model needs adjusting, but nothing majorly wrong.
Why is my Rfree hanging so high?  Is it becaues I have bad karma?  Is it to do with the twinning?  I am pretty sure my space group is right (based on the above info and the maps look fine; Phaser could not find a solution in P43).  There isn't room for anything else in the cell.
Any opinions or suggestions are most welcome as I wait for better xtals to grow...
Thanks in advance,
Christina


      
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