Interaction of "rigid" quantum systems
  We introduce the notion of a "rigid" quantum system as a system with constant relative positions of its nuclei and constant relative distribution of the electrons with respect to the nuclei.   In accordance with this definition, a molecule which does not interact with other objects, is a "rigid" quantum system. Molecule is also "rigid" if it interacts with other objects, but the interaction does not change the intrinsic structure of the molecule (or this change can be neglected).   Several "rigid" quantum systems interact one with another in the quantum manner. The interaction is ruled by the Schr{\"o}dinger equation [1] written for all the particles of the systems under consideration. We consider the case when the external potential is zero. 
A. A. Kolpakov, A. G. Kolpakov
math-ph math.MP