MOLECULAR-GEOMETRY OF INCOMMENSURATE STRUCTURES

Molecular-geometry calculations in the superspace framework of incomme nsurate structure analysis are presented in detail. In particular, a m ethod is described for attaching atoms to groups of others without the necessity of introducing further modulation functions. Modified struc ture-factor expressions are derived to simplify the application of con straints to modulation functions by introducing internal coordinate re ference points in a more general way. The consequences of such changes are discussed. The results can be useful to improve the existing algo rithms for incommensurate structure refinement and the analysis and pr esentation of results, especially for recently discovered incommensura te structures of large organic molecules. Some ambiguities still exist ing in the superspace approach related to molecular geometry are clari fied. The results may also be useful for the compilation of a database describing incommensurate structures.