Ab initio direct phasing in macromolecular crystallography: an applicationof the z-test

The z-criterion has been recently formulated as a tool for judging about th e ab initio solvability of a crystal structure via direct methods. The crit erion is reconsidered to take into account the recent powerful techniques o f phase refinement in direct and reciprocal space. A report is made on a me dium size crystal structure, recently solved by SIR97 by the pure applicati on of the tangent formula.