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In quantum mechanics, the Born–Oppenheimer approximation is a method for approximating the energy a quantum mechanical system by identifying “light” and “heavy” degrees of freedom and then treating the solving for the “light” dynamics as if depending only on the static configuration of the “heavy” degrees of freedom.
Historically this is motivated from, and still heavily used in practice, for the computation of energy spectra of molecules?, where the atomic nuclei? are much heavier than the electrons, so that their dynamics can be split off to a good degree of approximation.
The original reference is
An early textbook-like account is
An review is for instance in section 2.2 The Born-Oppenheimer approximation of