ON THE AB-INITIO SOLUTION OF THE PHASE PROBLEM FOR MACROMOLECULES AT VERY-LOW RESOLUTION - II - GENERALIZED LIKELIHOOD-BASED APPROACH TO CLUSTER DISCRIMINATION

The multisolution strategies for direct phasing at very low resolution : such as the few atoms model technique, result in a number of alterna tive phase sets, each of them arising from a cluster of closely relate d models. Use of a Monte-Carlo type computer procedure is suggested to choose between the possible phase sets. It consists of generating a l arge number of pseudo-atom models inside the mask defined by a trial p hase set and the use of histograms of magnitude correlation to evaluat e the masks. It is shown that the procedure may be considered as a gen eralization of the statistical maximum-likelihood principle and may be used as a powerful supplementary tool in the Likelihood-based approac hes to the phase problem solution.