where c is the speed of light, and B e = h(4πcI e ) -1 = h(4πcµr 2 e ) -1 is the rotational constant corresponding to the structure at the equilibrium with distance r e . The average displacement in the harmonic approximation is zero. Therefore, the vibrationally-averaged rotational constant at the harmonic approximation is equal to the equilibrium rotational constant. However, the asymmetric terms of the PES Taylor expansion may introduce a vibrationallyaveraged displacement. For example, due to the term V 3 ξ 3 /6 we have