              Residual factor R~cullis~ for centric reflections in this\
               derivative in this shell.\
\
                           sum| |Fph~obs~ +/- Fp~obs~| - Fh~calc~ |\
               R~cullis~ = ----------------------------------------\
                                   sum|Fph~obs~ - Fp~obs~|\
\
               Fp~obs~  = the observed structure factor amplitude of the native\
               Fph~obs~ = the observed structure factor amplitude of the\
                          derivative\
               Fh~calc~ = the calculated structure factor amplitude from the\
                          heavy atom model\
\
               sum is taken over the specified reflections\
\
               Ref: Cullis, A. F., Muirhead, H., Perutz, M. F., Rossmann, M. G.\
                    \& North, A. C. T. (1961). Proc. R. Soc. A265, 15-38.