Born–Oppenheimer approximation

Summary

The Born–Oppenheimer approximation separates the motion of the electrons and of the nuclei in a molecule. One first solves the electronic problem with the nuclei held fixed, which gives an energy $E(R)$ for each nuclear configuration $R$, the potential energy surface. Then the nuclei move as quantum particles in the potential $E(R)$. It works because nuclei are thousands of times heavier than electrons, and it is the basis of almost all of quantum chemistry.