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A general scheme for the improvement of electron-density maps is described which combines information from real and reciprocal space. The use of Sayre's equation, solvent flattening and histogram matching within this scheme has been described previously Main (1990). Acta Cryst. A46, 372-377. Non-crystallographic symmetry averaging, the use of a partial structure and constraints on individual structure factors have now been added. A computer program, SQUASH, is described which applies all these constraints simultaneously. Its application to the maps of several structures has been successful, particularly so when non-crystallographic symmetry is present. Uninterpretable maps have been improved to the point where a significant amount of the structure can be recognized. Applying the constraints simultaneously is more powerful than applying them all in series.
Cowtan et al. (1993) studied this question.