Theoretical and experimental advances in the techniques available for solving crystal structures have led to the development of several powerful techniques for solving complex structures, including those of proteins. In this 1995 book, Michael Woolfson and Fan Hai-Fu describe all the available methods and how they are used. In addition to traditional methods such as the use of the Patterson function and isomorphous replacement, and the direct methods, the authors include methods that use anomalous scattering and observations from multiple-beam scattering. The fundamental physics and mathematical analyses are fully explained. Practical aspects of applying the methods are emphasised.

The phase problem is reduced to a sign problem once the anomalous-scatterer or the replacing-heavy-atom sites are located.OASISadopts theCCP4 format [Collaborative Computational Project, Number 4 (1994).Acta Cryst.D50, 760–763]. It applies a direct-method procedure to break the phase ambiguity intrinsic to one-wavelength anomalous scattering (OAS) or single isomorphous replacement (SIR) data.

A procedure to combine the information from an electron micrograph (EM) and the corresponding electron diffraction (ED) pattern is proposed. Here the ED data will be used to obtain a set of amplitudes of the structure factors, while the EM will be used to obtain a set of starting phases assuming the weakphase-object approximation. A direct method is then used to extend the phase information from a resolution of about 2 to 1 A. The efficiency of the procedure has been verified by the test calculation on the model structure of copper perchlorophthalocyanine.

A new method is proposed to estimate the defocus (Af) from a single electron micrograph (EM). The method has been tested by simulations using theoretical EM's calculated under different defocus conditions. The preliminary method is successful except when the EM is taken near the optimum defocus. This can be improved by making use of the information from the electron diffraction pattern. The method will be effective for radiation-sensitive materials.

SAPI is a CCP4-format program that applies a direct-methods procedure to ®nd heavy-atom sites using single-wavelength anomalous dispersion (SAD) or single-isomorphous replacement (SIR) data. The program is designed to run in a fast (typical CPU time consumed is less than 1 min) and simple way. User input is kept to a minimum and the default parameter values are adequate in most cases. It can also be used to solve general small-molecule structures.