Density constraints and low-resolution phasing

Direct phasing needs additional information of a non-specific kind in order to select the correct phase set from all possible ones. This paper analyse s the use of constraints which can be formulated in terms of electron-densi ty values. One- and multi-dimensional histograms and connectivity propertie s are implemented as such constraints in density-modification procedures. T hese approaches usually cannot unambiguously select the best solution from a set of alternative phase variants. Nevertheless, they do allow the reject ion of wrong solutions and the use of cluster analysis and averaging on the remaining variants provide a good starting point for further phase-refinem ent procedures.