The Born-Oppenheimer (BO) approximation is a cornerstone of the theoretical treatment of molecular processes. It separates nuclear and electronic motion, splitting up an immensely challenging many-body problem into two highly involved but well-understood ones. The electronic problem, solved for fixed nuclear geometries, yields eigenvalues of the electronic Hamiltonian as a function of the nuclear coordinates. Each eigenvalue (as a function of the nuclear coordinates) forms a potential energy surface (PES) for a given electronic eigenstate. The set of these electronic .....