Map self-validation: a useful discriminator of phase correctness at low resolution

A new map-validation procedure is based on the correlation-coefficient agre ement between the observed structure-factor magnitudes and their extrapolat ed values from suitably modified electron-density maps from which they have been each in turn systematically excluded. The correlation coefficient ten ds to a maximum as the phase errors in a map are reduced. This principle wa s used to resolve the single-wavelength anomalous scattering (SAS) and sing le-derivative isomorphous replacement (SIR) phase ambiguity for a number of error-free trial structures. Applications employing real data sets tend to be more difficult owing to data incompleteness and errors affecting the co nstruction of the Argand diagram.