Born–Huang approximation

The Born–Huang approximation is an approximation closely related to the Born–Oppenheimer approximation. It takes into account diagonal nonadiabatic effects in the electronic Hamiltonian better than the Born–Oppenheimer approximation. Despite the addition of correction terms, the electronic states remain uncoupled under the Born–Huang approximation, making it an adiabatic approximation. The approximation is named after Max Born and Huang Kun who wrote about it in the Dynamical Theory of Crystal Lattices.