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 analyzing spectral data,
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(2015), 143, pp. 164104. 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.