On the geometrical features of molecular crystal packing in cubic, hexagonal, and trigonal systems

The geometric aspects of molecular crystal packing in cubic and hexagonal crystal systems are examined. It is shown that, during the packing of asymmetric molecules into a crystal, one of the key geometric factors influencing the prevalence of a particular symmetry group is the proportion of forbidden regions (excluded volume) within the unit cell where molecules cannot be accommodated. Symmetric molecules predominantly occupy special positions, which correspond to dense or closest sphere packings. A list of regular systems of space groups in cubic and hexagonal systems has been derived, whose regular point systems can describe dense molecular packings in terms of closest sphere packings.