The Born-Oppenheimer approximation fails when two electron states almost cross, also known as an "avoided crossing." An avoided crossing is illustrated in the above plot of adiabatic electronic energy as a function of R. (Figure by Prof. Troy Van Voorhis.)
5.73 covers fundamental concepts of quantum mechanics: wave properties, uncertainty principles, Schrödinger equation, and operator and matrix methods. Basic applications of the following are discussed: one-dimensional potentials (harmonic oscillator), three-dimensional centrosymmetric potentials (hydrogen atom), and angular momentum and spin. The course also examines approximation methods: variational principle and perturbation theory.