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