A new canonical perturbation procedure for studying nonadiabatic dynamics

We propose a new canonical perturbation procedure, based on Van Vleckʹs theory, for investigating nonadiabatic (i.e., non Born–Oppenheimer) dynamics. The basic idea is to let the unitary transformations depend on the matrix representations of angular momentum operators. The new procedure is applied to a simple model with two 2-dimensional diabatic electronic surfaces coupled by a λq term, which to some extent mimicks the bending and antisymmetric stretching degrees of freedom of NO2 in the ground and first excited electronic states. It is shown that the anharmonicities induced in the lowest surface by the coupling to the excited one are very precisely obtained from the proposed procedure.