Residual factor R for all reflections that satisfy the resolution
limits established by _refine.ls_d_res_high and
_refine.ls_d_res_low.
sum|Fobs - Fcalc|
R = ---------------------
sum|Fobs|
Fobs = the observed structure-factor amplitudes
Fcalc = the calculated structure-factor amplitudes
sum is taken over the specified reflections