RESFree f :

Reflections at a resolution lower than f will not be allowed to enter the free R-factor set (ie, all low resolution reflections will belong to the working set during minimisation). The reason for the presence of this keyword is that in some cases (see structures with strong periodicities, for example 4-$\alpha$-bundles), there may be a few low resolution reflections of outstanding intensity. The presence of these reflections in the working set can significantly reduce the parameter space that we have to explore (because only a small part of the orientations and translations can account for these outstandingly strong reflections). For this reason, we would not want to risk ``loosing'' these reflections in the test set, and so here comes RESF. Please feel free to consider this as an outrageous R-free biasing keyword (and dangerous practice, etc.).