The mean phasing power P for centric reflections in this
derivative.
sum|Fhcalc2|
P = (----------------------------)1/2
sum|Fphobs - Fphcalc|2
Fphobs = the observed structure factor amplitude of the
derivative
Fphcalc = the calculated structure factor amplitude of the
derivative
Fhcalc = the calculated structure factor amplitude from the
heavy atom model
sum is taken over the specified reflections