Upgrading the twin variables algorithm for large structural

Phase extension from lower to higher resolution by using an upgraded TWIN v ariables algorithm [Hountas & Tsoucaris (1995). Acta Cryst. A51, 754-763] i n protein molecules with close to 1000 non-H atoms is presented. Three poin ts of this procedure are of particular interest. (i) The use of a set of au xiliary variables providing a satisfactory fit for many kinds of constraint s: the new algorithm works efficiently despite the extreme 'dilution' of ve ry limited initial phase information into a much larger set of auxiliary va riables. (ii) The extension of this auxiliary variables set beyond the reso lution of the observed data, which enhances the phase extension in a so-cal led 'super-resolution' sphere. (iii) The use of the crystallographic symmet ry as a new figure of merit and as a reliable test for the correctness of t he phase-extension process allows an efficient screening.