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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 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).