Internal Degrees of Freedom of Molecules

For the internal dynamics of molecules it is of crucial importance that nuclei are much heavier than electrons. This leads to the adiabatic picture of the nuclei motion with respect to the motion of electrons. In the adiabatic picture, the coordinates of nuclei play the role of external parameters with respect to the state of the electronic sub-system, that is they play the same role as the adiabatic parameter λ in our consideration of the adiabatic process. Here, however, we are dealing with more subtle situation where nuclei coordinates behave as external parameters with respect to the electrons only, not to each other. Mathematically, the adiabatic picture implies the following structure of the molecule wavefunction Ψ(r,R) = ψ(R)ΦR(r) , (1)