Crystals and clusters: quaternions and cosines

Some aspects of the theory of arrays of rigid molecules are reviewed. Attention is focused on the orientation of the molecules. Quaternions are useful for non-linear molecules in determining the structure of the arrays and they may be adapted to give convenient vibrational displacement coordinates. Direction cosines play a similar role for linear molecules. The application of these ideas to molecular crystals is discussed and a preliminary analysis of molecular clusters is given.