A new approach to perturbation theory for breakdown of the Born-Oppenheimer approximation

Abstract

A new method for applying perturbation theory to correct for breakdown of the Born-Oppenheimer approximation is developed. The method does not involve an expansion in terms of excited vibronic states, but gives the correction to the wavefunction from each excited electronic state directly in terms of purely electronic matrix elements. It automatically includes all contributions from bound and continuum states. An efficient algorithm for solving the resulting differential equations is given. The method is tested on the non-adiabatic coupling in H₂ ⁺ and in the mesic molecule (H_μH)⁺.