Weighted residual factor wR 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, and that were used as the working
               reflections (i.e. were included in the refinement) when the
               refinement included the calculation of a 'free' R factor.
               Details of how reflections were assigned to the working and
               test sets are given in _reflns.R_free_details.
                    ( 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