In quantum chemistry, the Dunham expansion is an expression for the rotational-vibrational energy levels of a diatomic molecule:
where  and  are the vibrational and rotational quantum numbers, and  is the projection of  along the internuclear axis in the body-fixed frame.
The constant coefficients  are called Dunham parameters with  representing the electronic energy. The expression derives from a semiclassical treatment of a perturbational approach to deriving the energy levels. The Dunham parameters are typically calculated by a least-squares fitting procedure of energy levels with the quantum numbers.