The relative simplicity of the ground state PES near the minima even allows us to fit the atomic interactions with simple functions. When quantum effects play too big of a role, we must step up our treatment of Eq. (3), which must be reformulated beyond the Born-Oppenheimer approximation to account for multiple states and their couplings. Often, the time evolution of the nuclear wave function n can be represented by ensembles of mixed quantum-classical trajectories. 3 When Theory Came First: Case Studies 3.1 Bioinorganic Chemistry