Not all the diffracted photons from a crystal end up in the Bragg reflections from specified (hkl) planes. Indeed, for quite a large number of macromolecular crystals the non-Bragg diffraction or diffuse scattering is strong in intensity. The diffuse scattering is due to a breakdown in the periodicity of the crystal and carries information on the mobility and flexibility of the molecules in the crystal. There are text-books describing diffuse scattering from small molecule crystals such as Amorós and Amorós (1968) and Wooster (1962).

At the synchrotron the long exposure times used in the measurement of the high resolution Bragg data on film or IP also automatically give the diffuse scattering and reveal a diversity of diffuse background patterns from different crystals. These observations have stimulated considerable interest in trying to understand and interpret these features. Of great interest to the molecular biologist is the relationship between macromolecular structure and function. Recent years have shown that besides the static/time-averaged structural information, appreciation of the molecular flexibility and dynamics is essential. Usually this information has been derived from the crystallographic atomic thermal parameters and also from molecular dynamics simulations (see, e.g., McCammon (1984)) which yield individual atomic trajectories. A characteristic feature of macromolecular crystals compared to small molecule crystals, however, is that their diffraction patterns extend to quite limited resolution even employing SR.