Map self-validation: improved criteria to resolve the SIR or SAS phase ambiguity

A procedure was recently described that used the correlation coefficient (CC) agreement between the observed |F_h| and their associated unbiased `omit map' extrapolated values |X_h| from an initial trial map as the basis for resolving the SIR or SAS phase ambiguity. It is noted here that a significant improvement in selectivity can be obtained if this agreement is expressed in terms of the complex-valued F_h and X_h. A new scheme is outlined to exploit the weighted average of the two SIR or SAS phase choices. This procedure requires six FFTs per phase compared with three for the older method that randomly selected either of the two permitted phase choices from the Argand diagram as starting values. Trial calculations are encouraging for applications as low as 4 Å resolution.