Direct methods and structures showing superstructure effects. IV. A new approach for phase solution

The mathematical model proposed in paper III of this series [Cascarano, Giacovazzo & Luic (1988). Acta Cryst. A44, 176-183] for describing structures with superstructure effects has been used to derive probabilistic formulas for estimating triplet invariants. The formulas obtained proved sufficiently robust to be applied successfully to a wide range of structures with superstructure effects, in which deviations both of replacive and of displacive type from ideal pseudotranslational symmetry occur.