The Shift_and_Fix procedure in EXPO: advances for solving ab initio crystal structures by powder diffraction data

The Shift_and_Fix procedure is a new method which has been developed for improving the quality of a structure model obtained by the ab initio solution process from powder diffraction data. The main features of the new approach, which is fully automatic, are as follows: (a) the structure model usually attained at the end of the phasing process by direct methods is shifted partly and randomly; (b) a combination of Fourier map calculation and least-squares cycles has been designed for relocating the shifted atoms onto positions which can finally be moved onto the true ones by the standard model optimization approaches; (c) the Fourier map is calculated using coefficients which depend on the chemical content of the compound under study. When the figure of merit for selecting the best set of phases derived by direct methods does not work well, the ALLTRIALS strategy can be applied: it aims to investigate, automatically and sequentially, all the stored direct methods phasing sets and pick up the correct solution. The Shift_and_Fix method has been applied for improving the structure model calculated by each one of the phasing sets processed by ALLTRIALS. It has been implemented in the computer program EXPO and proved to be effective in providing a better ALLTRIALS outcome and increasing the probability of succeeding in the ab initio powder solution.