Weighted residual factor wR for reflections that satisfy the
               resolution limits established by _refine_ls_shell.d_res_high
               and _refine_ls_shell.d_res_low and the observation limit
               established by _reflns.observed_criterion, and that were used
               as the test reflections (i.e. were excluded from 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