Has anyone looked theoretically at how ncs-restraints affect the expected Rfree/R ratio?
Tickle et al., Acta Cryst. (1998). D54, 547-557 concluded Rfree/R = sqrt(Nobs/(Nobs-Nparam)) . He suggested that, with restrained refinement of coordinates plus individual isotropic B-factors, the effective number of parameters per atom is two. If we add strong N-fold NCS restraints on coordinates and B-factor, does that effectively reduce the number of parameters by a factor of N? Giving 2/N for parameters per atom? I'm curious how much of the drop in the r-free ratio observed on enforcing NCS is due to the reduction in the effective number of parameters, and how much is due to linking reflections in the free set with the working set. Given an expression to predict the effect of reducing number of parameters, seeing how much of the actual drop in Rfree/R it accounts for would let us see how severe the linkage problem is. Ed
