An analysis of the role of the Born--Oppenheimer approximation in calculating rotational--vibrational interactions in molecules

It is argued that whether the use of the Born--Oppenheimer approximation is thought to require consideration of the potential energy surface in the context of a full Coulomb Schrödinger Hamiltonian in which translational and rotational motions are explicitly considered, and then it is inconsistent to treat that surface without allowing for the rotational motion of the molecule. Some of the implications of this upon the calculation of partition functions are considered.