The correlation coefficient between the observed and
               calculated structure factors for reflections included in
               the refinement.
               The correlation coefficient is scale-independent and gives
               an idea of the quality of the refined model.
                            sumi(Foi Fci - <Fo><Fc>)
  Rcorr = ------------------------------------------------------------
            SQRT{sumi(Foi)2-<Fo>2} SQRT{sumi(Fci)2-<Fc>2}
               Fo = observed structure factors
               Fc = calculated structure factors
               <>   denotes average value
               summation is over reflections included in the refinement