2. Separation of variables in a molecular system

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.