Separation of variables in a molecular system
Separation and contraction of variables in molecular spectroscopy

We've just outlined the theoretical framework we need to describe molecular quantum physics. We will now implement it to calculate the energy levels of a molecule. As a first step, we'll try to simplify the approach, by separating the degrees of freedom of the molecular system, in accordance with Descartes' second precept: "to divide each of the difficulties I examine, into as many parcels as it is possible and necessary to solve them better". Wherever this results in an approximation, we will emphasize the limits of the description.