The correlation coefficient between the observed and
calculated structure factors for reflections included in
the refinement. This correlation factor is found in the
fitting using the Levenberg-Marquardt algorithm to search
for the minimum value of chisquare. Almost all computer
codes for Rietveld refinement employ the Gauss-Newton algorithm
to find parameters which minimize the weighted sum of squares
of the residuals.
A description of the equations is given on
http://www.water.hut.fi/tkarvone/fr_org_s.htm