The application of direct methods to non-centrosymmetric structures containing heavy atoms

Direct methods are applied to the difference structure factors for a structure containing one or more heavy atoms in known positions. The present procedure is initiated by subtracting the known heavy-atom contribution from the observed structure factor (assuming that the observed and calculated structure factors have the same phase) to obtain the magnitude and phase of the light-atom contribution. The Σ₂ phase relationship (tangent formula) is used to recalculate the phases of the light-atom contributions, and - consequently - to recalculate the magnitude of the light-atom contribution. An iterative procedure is used to optimize the phases and amplitudes before a difference Fourier map is calculated. The method is applicable also for the solution of partially known structures.