Upgrading the twin variables algorithm for large structures

Phase extension from lower to higher resolution by using an upgraded TWIN variables algorithm [Hountas & Tsoucaris (1995). Acta Cryst. A51, 754-763] in protein molecules with close to 1,000 non-H atoms is presented. Three points of this procedure are of particular interest. (i) The use of a set of auxiliary variables providing a satisfactory fit for many kinds of constraints: the new algorithm works efficiently despite the extreme 'dilution' of very limited initial phase information into a much larger set of auxiliary variables. (ii) The extension of this auxiliary variables set beyond the resolution of the observed data, which enhances the phase extension in a so-called 'super-resolution' sphere. (iii) The use of the crystallographic symmetry as a new figure of merit and as a reliable test for the correctness of the phase-extension process allows an efficient screening.