Structure determination in the presence of twinning by merohedry

This chapter details strategies for determining structures based on intensity data suffering from twinning. Progress can often be made in the early stages of structure determination without taking the twinning into account, whereas proper refinement of structures must account for the effects of twinning. Complications that can occur when handling anomalous-scattering information are discussed. Opportunities are identified for more formal treatments of the problem of phasing twinned data. The facility with which twinning can be analysed and discriminated from local noncrystallographic symmetry during the final stages of analysis is emphasized. Keywords: twinning; merohedral twinning; structure determination