Hi everyone,

I have a P31 twin crystal, resolution 2.0, and with twin fraction 0.451. I use phenix.refine to refine the structure and with freeR set to 8%.

 
% phenix.refine data.sca model.pdb twin_law="h,-h-k,-l" xray_data.r_free_flags.generate=True xray_data.r_free_flags.fraction=0.08

Result:

Final: r_work = 0.1793 r_free = 0.2258 bonds = 0.011 angles = 1.557

REMARK   3   FREE R VALUE                     : 0.2258

REMARK   3   FREE R VALUE TEST SET SIZE   (%) : 3.67

REMARK   3   FREE R VALUE TEST SET COUNT      : 2013

After the refinement, it seems like my freeR set size reduced by half (3.67%). Is it due to the twin effect, or something is wrong with my refinement?

And if I use ccp4 program “scalpeak2mtz “ to convert the .sca to .mtz and with freeR fraction set to 8%. Then refine it with phenix.

% phenix.refine data.mtz model.pdb twin_law="h,-h-k,-l" 

Final: r_work = 0.1669 r_free = 0.1970 bonds = 0.010 angles = 1.406

REMARK   3   FREE R VALUE                     : 0.2022

REMARK   3   FREE R VALUE TEST SET SIZE   (%) : 7.80

REMARK   3   FREE R VALUE TEST SET COUNT      : 4326

It turns out, my test set is still 8%, and R/Rfree looks even better than the first one. So I am wondering which one is the correct one and why?

Thanks!

Lauren