Rotation vibration in diatomic

As the rotational energy increases, the internuclear distance increases, due to the centrifugal force, which produces an increase in the moment of inertia. This distortion causes the rotational energy to be less than that of the rigid rotor and it is necessary to add the correction $-hDJ^2(J+1)^2$, where the centrifugal distortion constant D is a small positive constant.