A simple but fundamental question in the context of Quantum Mechanics is the following: if an atom is composed of particles with positive and negative charges, why does it not collapse? The classical model predicts that the charges will attract each other indefinitely and that the total energy of the system will tend to −∞ in a very short time. However, this is not the behavior observed in reality. In the case of the hydrogen atom, which can be solved explicitly, the possible energies of the atom are bounded from below. In the case of systems with many particles, arguing from the hydrogen atom case is not very satisfactory, since the interactions are more complex, as phenomena such as electron screening or chemical bonding arise when considering several nuclei.