Residual factor Rkraut for general reflections for this
               derivative in this shell.
                          sum|Fphobs - Fphcalc|
               Rkraut = -------------------------
                                sum|Fphobs|
               Fphobs  = the observed structure-factor amplitude of the
                           derivative
               Fphcalc = the calculated structure-factor amplitude of the
                           derivative
               sum is taken over the specified reflections
               Ref: Kraut, J., Sieker, L. C., High, D. F. & Freer, S. T.
                    (1962). Proc. Natl Acad. Sci. USA, 48, 1417-1424.