Nonadiabatic hamiltonian for electronic–vibrational coupling

A new version of adiabatic theory enabling one to evaluate higher‐order nonadiabatic terms has been developed. One introduces a nonadiabatic Hamiltonian Ĥ na so that the total Hamiltonian is given by Ĥ = Ĥ 0 + Ĥ na , where Ĥ 0 describes the noninteracting electronic and nuclear subsystems, and Ĥ na describes the electronic‐vibrational coupling. The correction to the total energy is calculated. The theory is applied to vibrational Raman scattering of aromatic molecules. Nonadiabaticity is found to play a key role in vibrational Raman scattering.