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