On the importance of the Born–Oppenheimer approximation in intramolecular dynamics

The concept of molecular structure, fundamental for correctly configuring space in the problem of describing the discrete spectrum of intramolecular dynamics, can be introduced only in the Born–Oppenheimer approximation. Therefore, it is this approximation that allows formalizing the formulation of this problem. The symmetry properties of dynamics give rise to limitations in determining some types of internal motions, whose violation leads to incorrect physical conclusions.