Residual factor R 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 (i.e., excluded 
               from refinement) reflections when refinement included 
               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|F~obs~ 
               - F~calc~| R = --------------------- sum|F~obs~| 
               F~obs~ = the observed structure factor amplitudes 
               F~calc~ = the calculated structure factor amplitudes 
               sum is taken over the specified reflections