[phenixbb] phenix real space refine & free R reflections

Hillen, Hauke hauke.hillen at mpibpc.mpg.de
Fri May 5 02:01:36 PDT 2017

Dear Phenix bb,

I have a rather conceptual question regarding the treatment of free R flags in phenix.refine and phenix.real_space_refine. I am refining an xray structure using phenix.refine (but not using the real space refinement implemented there) and at some point decided to play a bit with the standalone program phenix.real_space_refine. In particular, I was interested in testing real space refinement against different maps (experimental map vs. model-phased). Being used to the excellent handling of free R flags in Phenix, I assumed giving phenix.real_space_refine the output MTZ of a previous phenix.refine run as a map would be safe to do. But I then saw the phenix.refine FAQ states the following: 

"Are R-free flagged reflections included in the maps used for real-space refinement?
No, this is almost guaranteed to bias R-free; these reflections are removed internally prior to map calculation. However, the output maps will include these reflections unless you explicitly request otherwise.”

While it makes sense to me that including the free reflections in the maps used for real space refinement will bias R-free, I wonder why the default behavior of phenix.refine is to output maps which include these reflections. I assume most users will use these output maps (for example via the OPEN IN COOT button in the GUI) to iteratively improve their model by local real space refinement in Coot. Conceptually, how is real space refinement performed by phenix.refine (where the free R reflections are not used to calculate the map to refine against) different from manual local real space refinement in Coot? Will the latter not also bias R-free when the default output maps from phenix.refine are used?


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