The program ROTAX derives non-merohedral twin laws by identifying poorly fitting data during refinement and by transforming their indices by rotations or roto-inversions about possible direct- and reciprocal-lattice directions. Operations that transform these indices to integers are identified as possible twin laws.

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The derivation of non-merohedral twin laws during refinement by analysis of poorly fitting intensity data and the refinement of non-merohedrally twinned crystal structures in the program CRYSTALS (R. I. Cooper, R. O. Gould, S. Parsons and D. J. Watkin)