An enveloped cross rotation function

A procedure has been developed for the computation of the cross rotation function in the initial step of molecular replacement. This procedure involves the truncation of the model Patterson map by means of an envelope which follows the shape of this Patterson. Test calculations carried out using bovine phospholipase A2 as the search model and a mutant porcine phospholipase A2 data set as the target crystal with this enveloped cross rotation function indicate that the resulting orientation is nearly as accurate as that found after Patterson correlation refinement of the solution of the cross rotation function.