Once we have electronic densities and energies, we look back to pick up all the effects left out in the long chain of approximations we used to solve the electronic problem. Then, computational chemists will work to recover relativistic effects, nonadiabatic couplings, and interactions with external fields. Next, we can focus on the nuclear problem, Eq. (3). However, even before attempting to solve this equation, we can learn much about nuclear distribution by mapping the PES topography. Their minima will reveal equilibrium structures, and their maxima will tell us about transition states. Much of computational chemistry focuses on finding pathways on those surfaces to describe chemical reactions. The quest to solve the nuclear equation starts by realizing that near the minima, PES are nearly multidimensional parabolas. This allows us to analyze the vibrational nuclear motion in terms of a collection of normal (in the sense of orthogonal) harmonic modes.