It takes into account diagonal nonadiabatic effects in the electronic Hamiltonian better than the Born–Oppenheimer approximation.
The Born–Huang approximation asserts that the representation matrix of nuclear kinetic energy operator in the basis of Born–Oppenheimer electronic wavefunctions is diagonal: The Born–Huang approximation loosens the Born–Oppenheimer approximation by including some electronic matrix elements, while at the same time maintains its diagonal structure in the nuclear equations of motion.
Under the Born–Huang approximation, the Schrödinger equation of the molecular system simplifies to The quantity
serves as the corrected potential energy surface.
The value of Born–Huang approximation is that it provides the upper bound for the ground-state energy.