Separation of slow motion in E⊗ϵ vibronic states

Abstract A new separation of slow motion is introduced to describe linear as well as linear-plus-quadratic E ⊗ϵ vibronic states. Instead of separating the nuclear radial and angular coordinates altogether, only the radial nuclear coordinate is separated as parametric variable. It is shown that eigenstates of the new Hamiltonian are no longer involved in a degeneracy, and therefore the Born-Huang and Born-Oppenheimer approximations work well.