11.10 Harmonic Vibrational Analysis

11.10.4 Localized Mode Vibrational Analysis

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,416, 414, 415 interpreting two-dimensional vibrational spectra,344 or improving calculations that go beyond the harmonic approximation.169, 702, 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
       Controls whether a vibrational mode localization calculation is performed.
TYPE:
       INTEGER
DEFAULT:
       0 Normal mode calculation.
OPTIONS:
       1 Localized mode calculation with a Pipek-Mezey like criterion. 2 Localized mode calculation with a Boys like criterion.
RECOMMENDATION:
       None

LOCALFREQ_THRESH
       Mode localization is considered converged when the change in the localization criterion is less than 10-LOCALFREQ⁢_⁢THRESH.
TYPE:
       INTEGER
DEFAULT:
       6
OPTIONS:
       n User-specified integer.
RECOMMENDATION:
       None

LOCALFREQ_MAX_ITER
       Controls the maximum number of mode localization sweeps permitted.
TYPE:
       INTEGER
DEFAULT:
       200
OPTIONS:
       n User-specified integer.
RECOMMENDATION:
       None

LOCALFREQ_SELECT
       Select a subset of normal modes for subsequent anharmonic frequency analysis.
TYPE:
       LOGICAL
DEFAULT:
       FALSE Use all normal modes.
OPTIONS:
       TRUE Select a subset of normal modes.
RECOMMENDATION:
       None

LOCALFREQ_GROUPS
       Select the number of groups of frequencies to be localized separately within a localized mode calculation. The size of the groups are then controlled using the LOCALFREQ_GROUP1, LOCALFREQ_GROUP2, and LOCALFREQ_GROUP3 keywords.
TYPE:
       INTEGER
DEFAULT:
       0 Localize all normal modes together.
OPTIONS:
       1 Define one subset of modes to localize independently. 2 Define two subsets of modes to localize independently. 3 Define three subsets of modes to localize independently.
RECOMMENDATION:
       None

LOCALFREQ_GROUP1
       Select the number of modes to include in the first subset of modes to localize independently when the keyword LOCALFREQ_GROUPS > 0.
TYPE:
       INTEGER
DEFAULT:
       NONE
OPTIONS:
       n User-specified integer.
RECOMMENDATION:
       Modes will be included starting with the lowest frequency mode and then in ascending energy order up to the defined value.

LOCALFREQ_GROUP2 and LOCALFREQ_GROUP3 are defined similarly.