As one can see, the anharmonic shift ∆ω is proportional to the square of the amplitude (l 2 ) and is also induced by the anharmonic terms of the PES Taylor expansion (Equation ( )). This dependence indicates larger deviations from the harmonic frequency at higher vibrational excitations. A similar situation is observed in the rotational spectroscopy, where rotational constants B can be considered as observables. For a diatomic molecule with the interatomic distance r and moment of inertia I = µr 2 , taking into account the relation