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,510, 508, 509 interpreting two-dimensional vibrational spectra,421 or improving calculations that go beyond the harmonic approximation.197, 856, 422 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
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
LOCALFREQ_THRESH
Mode localization is considered converged when the change in the localization
criterion is less than .
TYPE:
INTEGER
DEFAULT:
6
OPTIONS:
User-specified integer.
RECOMMENDATION:
None
LOCALFREQ_MAX_ITER
LOCALFREQ_MAX_ITER
Controls the maximum number of mode localization sweeps permitted.
TYPE:
INTEGER
DEFAULT:
200
OPTIONS:
User-specified integer.
RECOMMENDATION:
None
LOCALFREQ_SELECT
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
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
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:
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.