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1. Determine the space group and unit cell of the real structure.

2. Test for systematically strong intensities.

3. Determine the space group of the superposition structure (see examples).

4. Determine the superposition structure or at least the heavy atom positions(s) in .

5. Determine the complementary structure or the real structure.

5.1 Compute and discuss the Patterson function of the complementary structure, if this corresponds to the systematically weak reflection.

5.2 Apply `Direct methods' (see above).

5.3 Apply the method of `linear structure factor equations'.

5.4 Resolve the ambiguity of the superposition structure using chemical knowledge, such as minimum distance between atoms, knowledge of groups, coordination polyhedra etc.

**Copyright © 1981, 1998 International Union of
Crystallography**