PRISM: automated crystallographic phase refinement by iterative skeletonization

A phase-refinement procedure based on iterative skeletonization of electron density maps is presented. As with traditional solvent-flattening methods, refinement alternates between real-space and reciprocal-space representations of the scattering density. A pseudoatom list derived from the modified skeleton of an initial electron density map provides calculated structure-factor amplitudes and phases. Recombination with the observed F_obs values yields a new map that can serve as the starting point for another round of skeletonization. Tests using partial structures to provide starting crystallographic phases have shown this refinement procedure to have a significantly larger radius of convergence than solvent flattening.