title: The crystallographic symmetry test for the correctness of a set of phases.
abstract: One of the most important problems in the application of direct methods for large structures is to establish reliable consistency criteria for the correctness of a phasing trial. The introduction in the twin variables method [Bethanis, Tzamalis, Hountas, Mishnev & Tsoucaris (2000). Acta Cryst. A56, 105-111] of a new criterion based on the crystallographic symmetry consists of testing the phase extension and refinement algorithm by deliberately sacrificing the space-group-symmetry information in the auxiliary variable set then using its gradual re-appearance as a criterion for correctness. In the present paper, the crystallographic symmetry test has been used in the implementation of the twin algorithm in two different ways: (i) as an overall test throughout the iterations that is likely to reflect the correctness of the phasing procedure for each one of the extension trials in a macromolecular phasing environment; (ii) as a convenient criterion to determine the optimum cycle for freeing the initial phases used by the algorithm for the phase-extension procedure.
authors.affiliation: Physics Laboratory, Agricultural University of Athens, 75 Iera Odos Votanikos, Athens 118-55, Greece.
authors.forename: P
authors.initials: P
authors.lastname: Tzamalis
authors.forename: K
authors.initials: K
authors.lastname: Bethanis
authors.forename: A
authors.initials: A
authors.lastname: Hountas
authors.forename: G
authors.initials: G
authors.lastname: Tsoucaris
mesh_terms: D064431:3-Oxoacyl-(Acyl-Carrier-Protein) Reductase; D000429:Alcohol Oxidoreductases / Q000737:chemistry; D018360:Crystallography, X-Ray / Q000379:methods*; D008956:Models, Chemical; D011506:Proteins / Q000737:chemistry; D006163:Ribonuclease T1 / Q000737:chemistry
chemical_list: D011506:Proteins; D000429:Alcohol Oxidoreductases; D064431:3-Oxoacyl-(Acyl-Carrier-Protein) Reductase; D006163:Ribonuclease T1