On the application of phase relationships to complex structures. XXXIV. VFOM - a new figure of merit for protein phase sets at moderate resolution

In recent years it has been shown that direct methods are capable of solving the structures of small proteins. Mukherjee & Woolfson [Acta Cryst. (1993), D49, 9-12] have shown that useful phase sets can be produced even at 3 Å resolution but that the standard figures of merit could not distinguish the better phase sets from others. They found modified forms of the standard figures of merit that could pick out better phase sets for 2 Å resolution or higher. Gilmore, Henderson & Bricogne [Acta Cryst. (1991), A47, 842-846] have shown that evaluation of the log-likelihood gain, coming from entropy-maximization procedures, is also very successful in picking out good protein phases sets. A new figure of merit is described, based on the expected charactistics of an electron-density map for a protein, and comparisons are made with the other figures of merit mentioned above.