Residual factor Rcullis for centric reflections for this
derivative in this shell.
sum| |Fphobs +/- Fpobs| - Fhcalc |
Rcullis = ----------------------------------------
sum|Fphobs - Fpobs|
Fpobs = the observed structure-factor amplitude of the native
Fphobs = the observed structure-factor amplitude of the
derivative
Fhcalc = 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. London Ser. A,
265, 15-38.