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Keywords:

  • computer programs;
  • COSET;
  • twinning;
  • merohedry;
  • pseudo-merohedry

COSET is a program written in ISO C99 with POSIX extensions which uses left coset decompositions to determine possible merohedral and pseudo-merohedral twin laws. In addition to a stand-alone program, the code may be compiled as a Python extension module. The program can create SHELXL instruction files which incorporate the appropriate TWIN and BASF instructions for the possible twin law(s). COSET may also be directed to execute a locally installed copy of the SHELXL binary executable to test the candidate twin laws in trial refinements. This facilitates the quick screening and assessment of possible twin laws.