From Partial Structure to Completeness — Direct Methods Applied to Difference Structure Factors
When part of a structure is known, direct methods can be used to solve the unknown part of that structure. Often the known part of the structure consists of one or more heavy atoms, either on general or on special or pseudo-special positions. The known part of the structure may also consist of a molecular fragment, found by ab-initio direct methods or by Patterson vector search techniques.
The difference structure factors, phased by the partial structure, are used as input to a weighted tangent-refinement procedure for phase extension and for the refinement of input phases and amplitudes. If the known atoms do not uniquely determine the structure, symbolic addition techniques are introduced to solve the (pseudo-symmetry) ambiguities.
The method is referred to as ‘DIRDIF. It is most useful when the known part is only marginally sufficient to solve the structure, when the known atoms lie on special or pseudo-special positions (origin ambiguity), or when, for noncentrosymmetric structures, the known atoms form a centrosymmetric arrangement (enantiomorph ambiguity).
The computer program system is also referred to as ‘DIRDIF’ (Beurskens, Bosman, Doesburg, Gould, van den Hark, Prick, Noordik, Beurskens, Parthasarathi, Bruins-Slot, Haltiwanger, Strumpel, Smits, Garcfa-Granda, Smykalla, Behm, Schäfer, and Admiraal, 1990). DIRDIF consists of a collection of individual programs, written in FORTRAN77, which are designed to run fully automatically. The actual calculations are controlled by a control-data file which is generated by an interactive start-up procedure. A simple computer-dependent job-control procedure is used to activate various individual programs whenever their execution is requested by the control-data file.
Vector search rotation functions, and translation functions in reciprocal space, as well as heavy-atom Patterson interpretation procedures are incorporated in the program system.
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