We froze the atomic nuclei in order

We froze the atomic nuclei in order to compute the eigenenergies of the electronic Hamiltonian presented in Figure 1.3. This is an approximation.Unfreezing the nuclei gives rise to perturbations that couple different electronic states of the molecule and, in principle, spoils the quantum labels in Eq. (1.5). These perturbations arise from the relative radial motion of the nuclei and the rotational motion of the internuclear axis. For most diatomic molecules, these perturbations are generally weak - often on the order of a few cm-1, at best. It can be seen from Figure 1.3 that, when oxygen and hydrogen are brought together to the distance of a chemical bond length (~2-Bohr radii), the lowest-energy electronic state of OH is separated from other electronic states by >30 000 cm-1. At these interatomic distances, the few cm-1 couplings between different electronic states will have no noticeable effect on the lowest-electronic energy of OH, and the molecule can be thought of as residing in a single electronic state X21.