Direct Methods and Anomalous Disper-

The electron density function, Q(r), in a crystal determines its diffraction pattern, i.e. both the magnitudes and phases of its x-ray diffraction maxima, and conversely. If, however, as is always the case, only magnitudes are available from the diffraction experiment, then the density function ρ(r) cannot be recovered. If one invokes prior structural knowledge, usually that the crystal is composed of discrete atoms of known atomic numbers, then the observed magnitudes are, in general, sufficient to determine the positions of the atoms, i.e. the crystal structure. It should be noted here that the recognition that observed diffraction data are in general sufficient to determine crystal structures uniquely was an important milestone in the development of the direct methods of crystal structure determination. The erroneous contrary view, that crystal structures could not, even in principle, be deduced from diffraction intensities, had long been held by the crystallographic community prior to c. 1950 and constituted a psychological barrier which first had to be removed before real progress could be made.