title: Direct methods: a paradox with regard to the convergence of random phase trials toward solutions.
abstract: A frustrating observation, based on an R(min) variance analysis within the `shake and bake' framework of direct methods phasing, is described. The variance of R(min) can on occasion identify large subsets of phases that have a significantly lower mean phase error than the entire direct methods phase set of otherwise unsuccessful phasing trials for which the overall phase error occasionally dips below 75 or 80°. This is the first time, other than for a handful of Σ1 phase indications in optimal situations, that a priori phase estimates have been attained for large numbers of E values, prior to solving the structure. Although the a priori variance of R(min) is a useful tool for identifying such phases, the a posteriori phase refinement shifts indicated by its minimum often prevent a successful convergence to the solution. Similar efforts to encourage solution convergences in the realm of real space have also been discouraging.
authors.affiliation: Department of Structural Biology, Hauptman-Woodward Medical Research Institute, 700 Ellicott Street, Buffalo, NY 14203, USA. langs@hwi.buffalo.edu
authors.forename: D A
authors.initials: DA
authors.lastname: Langs
authors.forename: H A
authors.initials: HA
authors.lastname: Hauptman
mesh_terms: D000465:Algorithms; D000704:Analysis of Variance; D018360:Crystallography, X-Ray / Q000379:methods*; D015394:Molecular Structure; D011506:Proteins / Q000032:analysis* / Q000737:chemistry
chemical_list: D011506:Proteins