The harmonic approximation combined with the frequency scaling is a very robust, computationally affordable, and simple way for calculation of vibrational properties. For instance, the root-mean-squared deviation (RMSD) of the scaled harmonic frequencies from the experimentally-obtained fundamental transitions is only around 20 cm -1 at the B3LYP/def2-TZVP level of theory. However, the direct comparison of computed in this way theoretical values with experimental data may be of limited use, because real observables in the rotational or vibrational spectroscopy are generally anharmonic. For example, in the classical mechanics the angular vibrational frequency can be approximately expressed as