Tutorial on automated Patterson interpretation to find heavy atoms

The algorithms employed for automatic location of heavy atoms via the Patterson function in the programs SHELXS-86 and SHELXS-90 are discussed, and are illustrated in the form of two tutorial exercises. As a small molecule crystallographer participating in a macromolecular computing school, I have tried to choose a topic of importance to both small molecule and macromolecular crystallographers, namely the location of heavy atoms via the Patterson function. In the case of heavy atom derivatives of proteins this Patterson would be calculated using the isomorphous difference structure factors. At least a quarter of organic and inorganic structures are solved by the heavy atom method, and the preparation of heavy atom derivatives is almost obligatory for proteins if no suitable search model is available for molecular replacement. Despite this, there is a striking lack of user-friendly computer programs, general for all space groups and types of problem, for the automatic location of heavy atoms, in sharp contrast to the situation in small-molecule direct methods. This seems to be simply an historical accident; the necessary theory has be well understood for at least 40 years, and the computer resources required are modest.