In extended systems such as polypeptides, the normal modes are in general delocalized over the whole system. For the amide I band, the normal modes consist of combinations of the C=O stretching vibrations of several peptide groups. To reconstruct a simpler picture in terms of localized vibrations, one collects the subsets of modes that contribute to the vibrational band of interest (e.g., the amide I band), Q sub , and performs a unitary transformation ,