Nuclei are thousands of times heavier than electrons. While a nucleus moves a little, an electron travels many times through the molecule. A lot can be simplified when assuming electronic motion in a field created by immobile nuclei. This concept is behind what is called adiabatic approximation. We obtain two coupled equations: one for the motion of the electrons in the field of fixed nuclei, and the other for the motion of the nuclei in the potential averaged over the electronic positions. Only after this approximation is introduced can we obtain the fundamental concept of chemistry: the molecular structure in three-dimensional space. Also, for each electronic state we obtain the concept of potential energy surface (PES) for the motion of the nuclei. These surfaces may intersect, which leads to the idea of the conical intersections, crucial for chemical reactions.
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