Ill-conditioned Shake-and-bake: the trap of the false minimum

The alternation of phase refinement with the imposition of real-space const raints is the essence of the Shake-and-Bake procedure. Typically, these con straints prevent trial structures from falling into local minima. Neverthel ess, Pl structures appear to migrate to false minima with significant Frequ ency. These false minima are characterized by the presence of a large 'uran ium' peak on the corresponding Fourier map. Fortunately, they can be recogn ized and avoided by considering the values of the minimal function both bef ore and after the application of constraints. However, it appears that find ing solutions for large P1 structures is likely also to require parameter-s hift conditions different from those that have been found to work well in o ther space groups. In fact, these conditions often yield an unusually high percentage of solutions.