Figure 1 An Argand diagram illustrating the structure factor of a reflection with significant anomalous scattering contribution. The brown vector represents the contribution of normally scattering atoms, FN; the red and purple vectors relate the normal, FA, FA', and anomalous, ±FA'', contributions of the anomalously scattering substructure for two Friedel mates; the two black vectors show the total structure factors for each of the Friedel mates, FT+ and FT-; the blue vector shows the total average structure factor FT. In the SAD experiment only the amplitudes (lengths) of FT+ and FT- are available (the ends of black vectors must lie on the circles of appropriate radii), but if the anomalous substructure is known, the size and orientation of the red/purple vectors can be calculated. The purple ±FA'' vectors must end up on the corresponding circles. To calculate the electron-density map, the total structure factor should be used, with amplitude |FT| (|FT+| + |FT-|)/2 and phase T.

© International Union of Crystallography 2007