The computation of harmonic frequencies leads to molecular vibrations described by coordinates which are often highly de-localized. For larger molecules many vibrational modes can potentially contribute to a single observed spectral band, and information about the interaction between localized chemical units can become less readily available. In certain cases, localizing vibrational modes using procedures similar to the localized orbital schemes discussed previously in this manual can therefore provide a more chemically intuitive way of analysing spectral data,421, 419, 420 interpreting two-dimensional vibrational spectra,344 or improving calculations that go beyond the harmonic approximation.158, 726, 345 It is also possible to include only a subset of the normal modes in the localization calculation by invoking the LOCALFREQ_SELECT rem variable. This can be useful to improve convergence in larger molecules or to explore the coupling between specific vibrational modes. These modes are defined in the $alist block. Alternatively it is possible to localize high and low frequency modes separately in a single calculation using LOCALFREQ_GROUPS and related inputs.
LOCALFREQ_GROUP2 and LOCALFREQ_GROUP3 are defined similarly.