In quantum chemistry, the Dunham expansion is an expression for the rotational-vibrational energy levels of a diatomic molecule:
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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.[2] The Dunham parameters are typically calculated by a least-squares fitting procedure of energy levels with the quantum numbers.
Relation to conventional band spectrum constants
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This table adapts the sign conventions from the book of Huber and Herzberg.
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