The Born-Oppenheimer approximation and matrix elements of terms in the effective molecular Hamiltonian that violate the BO approximation. Vibration, rotation, spin-orbit, interelectronic interaction. Hund's coupling cases.

What happens in the vicinity of a curve crossing?

All coupling processes are mediated by vibrational overlap integrals, which are computed numerically. Semiclassical calculation of overlaps reveals physical factors controlling computed values...