CCP4i: Graphical User Interface
MIR Tutorial Bath - Phasing statistics appendix

Statistics of phasing
Most useful indicators of the validity of a heavy-atom site are the real
occupancy and the atomic thermal parameter(s) .

Reduction in R _{Cullis} when new site is added is also a good indicator.

R _{Cullis} =
<P -weighted lack of closure> / <Isomorphous difference>

But for d_{min} >
» 3.5Å
occupancies and thermal parameters are highly correlated
and the latter should probably not be refined.

To judge a derivative as a whole, check R _{Cullis} and the phasing power.

R _{Cullis} <
» 0.6 are
considered excellent, <
» 0.9 usable.

For anomalous data any R _{Cullis} is useful.

Phasing power measures the sharpness of the phase distribution (contribution
of derivative to phasing information compared with noise).

Phasing power = <Heavy-atom amplitude> / <P -weighted lack of closure>

> » 1.5 excellent, >
» 1.0 good, >
» 0.5 usable.
Note that all root mean-square (RMS) averages are very sensitive to outliers:
underestimates R _{Cullis} and phasing power, so straight means are safer.

Check mean & standard deviation of absolute phase difference <|D f |>
between F _{P} & F _{H}
(N.B. not applicable in SIR[AS] cases ).

If data accurately scaled, phase of F _{P} is uncorrelated with phase
of F _{H} .

<|D f |> should be
» 90°
(± 10°).

s should be
» 52° for acentric,
» 90° for centric.

Mean figure of merit measures precision of the "best" phase.

<FOM> »
mean of cosine(estimated phase error).