the absorption of the NH 3 fragment, with subsequent excitation of the bending and umbrella vibrations. These bands and effective modes, labelled 9, 10 and 11 in Fig. , confirm the analysis presented in the previous section. The analysis of the effective modes and the bands assignment just discussed are summarized in Table , where we report, in the right column, the average frequencies of the modes identified from their effective modes analysis. The left column of the table reports experimental IR data from Ref. . The modes assignment is presented in the central column, where we adopt the notation for the modes used in Ref. . Note that, in Ref. the bands were assigned based on harmonic normal modes calculations with electronic energy calculated at the Hartree-Fock level. In general, we find that the effective modes analysis captures many features of the experimental IR spectrum, even though quantitative agreement is lacking in some regions of the analyzed domain. It is plausible to assume that the deviations are mainly caused by either the force field used to generate the MD trajectory or by the strong anharmonic dynamics of alanine. However, we remind that the modes assignment of the experimental frequencies from Ref. relied on the harmonic approximation, thus some disagreement between normal modes and effective modes is to be expected.