Hi Ian (& ccp4bb'ers),NCS ties reflections in reciprocal space by the interference G-function effect. Nothing more. So you get an R-free value that is lower than if you don't have NCS. One should be aware of that, and referees should be aware of that.
I currently have a structure that has 12-fold NCS in the asymmetric unit. The free R-factor is lower than the R-factor. I expect that future referees will not view that kindly.
A number of people have suggested to use different approaches to get rid of this reciprocal space binding effect. One of these people (Bart Hazes I think, correct me if I'm wrong) suggests to take reflections for the R-free as thin shells in reciprocal space. The thin shell is omitted completely from the target for refinement (I suppose omitting a shell of data completely will also have deleterious effects on the refinement, I don't know by how much). Problem is, none of the data processing programs or suites of programs has implemented this as far as I know. A better approach would be to use the NCS operator (the transpose and the inverse of the rotation matrices in fact, including the identity matrix for all cases including the cases where the only NCS is 1-fold NCS, i.e. the presence of solvent in the asymmetric unit or unit cell) to select the subset of reflections that are going to be omitted from the refinement target: take one reflection, select all equivalents that are bound to it by the interference effect, and repeat the process until you have reached the required number of reflections to be omitted. But this requires serious programming... And someone willing to modify all data processing suites to include this approach. But that would satisfy referees because it is the only approach that is valid.
Fred. Ian Tickle wrote:
The problem is that real life is never simple! - and NCS really messes things up! Cheers -- Ian
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