Nonadiabatic effects in avoided crossings of electronic potentials: An application of the generator coordinate approximation

The generator coordinate approximation (GCA), a recently proposed nonadiabatic theory of molecular structure, is briefly reviewed. Results of this method, applied to a series of avoided crossings of electronic potentials in diatomics with different topologies, are presented. It is found that in all cases the GCA significantly improves the adiabatic vibrational energies. Large percentages of the nonadiabatic effects are removed. As the electronic energy gap at the crossing decreases, the GCA recovers a larger part of the nonadiabatic effect, and in the crossing limit, it becomes exact.