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.
sum|Fobs**2 - Fcalc**2|
R = ---------------------
sum|Fobs**2|
Fobs = the observed structure-factor amplitudes
Fcalc = the calculated structure-factor amplitudes
sum is taken over the specified reflections