Parameterization of the close packing of molecules in the unit cell

The box model of crystal packing describes unit cells in terms of a limited number of arrangements, or packing patterns, of molecular building blocks. Cell dimensions have been shown to relate to molecular dimensions in a systematic way. The distributions of pattern coefficients (cell length/molecular dimension) for thousands of structures belonging to P2₁/c, P, P2₁2₁2₁, P2₁ and C2/c are presented and are shown to be entirely consistent with the box model of crystal packing. Contributions to the form of the histograms from molecular orientation and molecular overlap are discussed. Gaussian fitting of the histograms has led to the parameterization of close packing within the unit cell and it is shown that molecular crystal structures are very similar to one another at a fundamental level.