|CCP4i: Graphical User Interface|
|MIR Tutorial Bath - Phasing statistics appendix|
Most useful indicators of the validity of a heavy-atom site are the real occupancy and the atomic thermal parameter(s).
Reduction in RCullis when new site is added is also a good indicator.
RCullis = <P-weighted lack of closure> / <Isomorphous difference>
But for dmin > » 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 RCullis and the phasing power.
RCullis < » 0.6 are considered excellent, < » 0.9 usable.
For anomalous data any RCullis 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 RCullis and phasing power, so straight means are safer.
Check mean & standard deviation of absolute phase difference <|Df|> between FP & FH (N.B. not applicable in SIR[AS] cases).
If data accurately scaled, phase of FP is uncorrelated with phase of FH.
<|Df|> 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).