Twinning Tools

1. ROTAX - written by Simon Parsons and Bob Gould of Edinburgh University. This program requires that the structure has been solved and an FCF structure factor file is available. It searches for possible twin-laws and gives a figure of merit for each such possible (generally 2-fold axis) twin law.
2. TwinRotMat - an option in PLATON. This does the same as ROTAX but uses a different algorithm.
3. Write HKLF5 File - for non-merohedral twins. Generally it is FAR preferable to treat these at the data-reduction stage. However, such twins may not be recognised early on, or it may be decided to continue with a single lattice integration. This utility is an ad hoc method of post-processing the reflection file. The important choice is to decide which reflections will overlap their twin component, either fully, partially or not at all. This can be done by consideration of three possible discriminator functions :
   • Delta function on index non-integrality
   • angular difference between the scattering vectors
   • difference between sin(theta)/lambda values
   The method is highly empirical and it is necessary to try several possibilities, refining model structures against the output HKLF5 files. If a pair of reflections have their discriminator function (df) less than parameter R1, they are treated as perfectly overlapped. If a pair of reflections have df greater than parameter R2, they are treated as non-overlapped. Between these two values they are partially overlapped and ignored by the program (on the assumption that the intensities must be seriously in error). Suitable twin law matrices may be read from the output files of ROTAX or TwinRotMat or may be input manually. The routine writes a SHELX format HKLF5 type file. See the SHELX manual for how to proceed from this stage.