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