Singularities in the Hamiltonian at electronic degeneracies Original Research Article

We discuss the singularities which arise in the Hamiltonian operator at a crossing seam involving two potential energy surfaces of the same global symmetry. The Mead–Truhlar and our own equations are discussed and found to differ from each other, although leading to identical phases up to a constant factor and sign in the vicinity of the crossing seam. Also established are the relations which link the vector and scalar gauge potentials with the mixing angle.