The correlation coefficient between the observed and
               calculated structure factors for reflections not included
               in the refinement (free reflections).
                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 not included
                in the refinement (free reflections)