One‐and‐a‐half wavelength approach

In many cases single‐wavelength anomalous diffraction (SAD) phasing leads to a successful structure solution, but it is impossible to predict beforehand if single‐wavelength data with a certain amount of anomalous signal resulting will be sufficient. It is therefore safer to continue collecting data at different wavelengths according to a MAD protocol, but to simultaneously attempt to phase the first data set by the SAD method. If this is successful, then further data collection can be abandoned. This `1.5‐wavelength' approach may save a substantial amount of time and effort and diminishes the effects of crystal radiation damage. The principles of SAD phasing are illustrated using vector diagrams in the Argand plane.