Structure analysis of displacively modulated molecular crystals

A structure-factor formalism for incommensurate modulated structures is derived. It allows for several simultaneous translational and rotational displacements of molecules or molecular segments, which are considered as being rigidly displaced. It incorporates treatment of several displacement waves in the crystal and makes full use of the four-dimensional symmetry description of de Wolff, Janssen & Janner [Acta Cryst. (1981), A37, 625-636]. A computer program based on the formalism has been applied to existing data sets on phenothiazine-TCNQ [Kobayashi (1974). Acta Cryst. B30, 1010-1017] and biphenyl [Baudour & Sanquer (1983). Acta Cryst. B39, 75-89], and to a new data set on [bis(ethylenedithio)-TTF]₂I₃ [Leung et al. (1984). J. Am. Chem. Soc. 106, 7644-7646].