Residual factor R(Fsqd) 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, calculated on the squares of the
observed and calculated structure-factor amplitudes.
sum|Fobs2 - Fcalc2|
R(Fsqd) = ---------------------------
sum|Fobs2|
Fobs2 = squares of the observed structure-factor amplitudes
Fcalc2 = squares of the calculated structure-factor
amplitudes
sum is taken over the specified reflections