Weighted residual factor wR for all reflections that satisfy the
               resolution limits established by _refine.ls_d_res_high and
               _refine.ls_d_res_low.
                    ( sum|w |Yobs - Ycalc|2| )1/2
               wR = ( ---------------------------- )
                    (        sum|w Yobs2|      )
               Yobs  = the observed amplitude specified by
                         _refine.ls_structure_factor_coef
               Ycalc = the calculated amplitude specified by
                         _refine.ls_structure_factor_coef
               w       = the least-squares weight
               sum is taken over the specified reflections