On Geometrical Features of Packing of Molecular Crystals of Cubic, Hexagonal, and Trigonal Syngonies

The geometrical aspects of packing of molecular crystals of cubic and hexagonal syngonies have been considered. It has been shown that, when asymmetric molecules are packed into a crystal, a fraction of the unit-cell regions forbidden for arrangement of molecules (forbidden volume) is one the main geometrical factors affecting the abundance of a particular group of symmetry. Symmetric molecules predominantly occupy particular orbitals, which correspond to the close and closest packings of spheres. A list of regular systems of space groups of symmetry of cubic and hexagonal singonies has been obtained, the regular systems of points of which can describe close packings of molecules in terms of the closest packings of spheres.