Residual factor R for reflections that satisfy the resolution
limits established by _refine.ls_d_res_high and
_refine.ls_d_res_low and the observation limit established by
_reflns.observed_criterion, and that were used as the test
reflections (i.e. were excluded from the refinement) when the
refinement included the calculation of a 'free' R factor.
Details of how reflections were assigned to the working and
test sets are given in _reflns.R_free_details.
sum|Fobs - Fcalc|
R = ---------------------
Fobs = the observed structure-factor amplitudes
Fcalc = the calculated structure-factor amplitudes
sum is taken over the specified reflections