We reiterate that when two or more potential energy surfaces approach each other, or even cross, the Born–Oppenheimer approximation breaks down, and one must fall back on the coupled equations.

What is breakdown of Born-Oppenheimer approximation?

The breakdown of the Born-Oppenheimer approximation: the effective vibration-rotation hamiltonian for a diatomic molecule. An effective vibration-rotation hamiltonian for the ground electronic state of a diatomic molecule is derived.

What is the significance of Born-Oppenheimer approximation and when does this approximation break down?

The Born-Oppenheimer Approximation is the assumption that the electronic motion and the nuclear motion in molecules can be separated. It leads to a molecular wave function in terms of electron positions and nuclear positions.