Organic structures, global optimization methods

In recent years an increasing number of crystal structures have been solved by application of pure or hybrid global optimization methods applied to powder diffraction data. The most striking applications regard organic structures particularly resistant to traditional methods. The first organic material of... 

Table 8.1 Recent examples of organic structures solved by global optimization methods applied to powder diffraction data.

This implies that any efficient search method must include structure relaxation (local optimization). We also note that all global optimization methods rely on the assumption that the reduced energy landscape will have a well-organized overall shape (Fig. 1), which is often true for chemical systems. ... 

DST methods are particularly competitive for organic compounds, which are more resistant to the traditional approaches and whose structural models can be easily guessed. At present, the complexity of crystal structures solved by direct-space methods is essentially limited by the number of DOFs that can be handled by the global optimization algorithms within a reasonable amount of time. In prospect, improvement of both search algorithms and computer power may overcome this limitation. The major pitfalls for the use of DST are (a) they are time consuming (b) they are dependent on the existence of reliable prior structural information. Partially incorrect models may compromise the success of the procedure independent of the computer time spent (c) they are sensitive to the accuracy of the peak profile parameterization through peak-shape and peak-width functions. ... 

DG was primarily developed as a mathematical tool for obtaining spahal structures when pairwise distance information is given [118]. The DG method does not use any classical force fields. Thus, the conformational energy of a molecule is neglected and all 3D structures which are compatible with the distance restraints are presented. Nowadays, it is often used in the determination of 3D structures of small and medium-sized organic molecules. Gompared to force field-based methods, DG is a fast computational technique in order to scan the global conformational space. To get optimized structures, DG mostly has to be followed by various molecular dynamic simulation. 

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