Quaternions in molecular modeling
  Quaternions are an important tool to describe the orientation of a molecule. This paper considers the use of quaternions in matching two conformations of a molecule, in interpolating rotations, in performing statistics on orientational data, in the random sampling of rotations, and in establishing grids in orientation space. These examples show that many of the rotational problems that arise in molecular modeling may be handled simply and efficiently using quaternions. 
Charles F. F. Karney (Sarnoff Corporation)