Born-Oppenheimer approximation

Born-Oppenheimer approximation is a technique used in quantum chemistry and condensed matter physics. Since the mass of the atomic nuclei is far greater than the mass of the electrons orbiting them, for a given energy, the electrons move much faster than the nuclei. Therefore, the positions of the nuclei can be considered to be approximately fixed from the electrons' perspective. The motion of the electrons can therefore be considered decoupled from the motion of the nuclei, which leads to the elimination of several terms from the Schrödinger equation. This is a good approximation, and has become a foundation for the physical study of solid and molecular systems.

The explicit consideration of the coupling of electronic and nuclear (vibrational) movement is known as electron-phonon coupling in extended systems or vibronic coupling in zero-dimensional systems.