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 analyses the use of constraints which can be formulated in terms of electron‐density values. One‐ and multi‐dimensional histograms and connectivity properties are implemented as such constraints in density‐modification procedures. These approaches usually cannot unambiguously select the best solution from a set of alternative phase variants. Nevertheless, they do allow the rejection of wrong solutions and the use of cluster analysis and averaging on the remaining variants provide a good starting point for further phase‐refinement procedures.