Determination of a phase angle

Interpretation of interatomic vectors. Use of known atomic positions for an initial trial structure (a preliminary postulated model of the atomic structure) can be made, by application of Equations 6.21,4 and 6.21.5 (Chapter 6), to give calculated phase angles. Methods for obtaining such a trial structure include Patterson and heavy-atom methods. Such methods are particularly useful for determining the crystal structures of compounds that contain heavy atoms (e.g., metal complexes) or that have considerable symmetry (e.g., large aromatic molecules in which the molecular formula includes a series of fused hexagons). The Patterson map also contains information on the orientation of molecules, and this may also aid in the derivation of a trial structure. 

Comparison of isostructural crystals. Phases can be estimated by comparing intensities of isomorphous (isostructural) crystals that differ only in the identity of one atom. This isomorphous replacement method is the method of choice for macromolecular (protein and nucleic acid) phase determination. In this case the isomorphism is generally between the crystalline macromolecule and its heavy-atom derivative obtained by replacing some of the solvent in the crystal by a compound containing a heavy atom. 

Measurement of multiple Bragg diffraction. Some sets of phases may be derived from experimental measurements of i/ -scan profiles of reflections exhibiting the effects of multiple Bragg diffraction. Groups of relative phases have absolute values that can be detected by this method which is currently in the developmental stage, and is only applicable to those sets of Bragg reflections that display the effect. 

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