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28
LOCAL CORRELATION TREATMENTS
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Usually, diffuse basis functions are important for obtaining accurate intermolecular interactions.
When diffuse basis sets are used, it may happen that the Pipek-Mezey localization does not yield
well localized orbitals. This problem can in most cases be overcome by using the directive
PIPEK,DELETE=n
as described in section 28.9.3
A final warning concerns local density fitting (see sections 28.10 and 11): local fitting must not
be used in counterpoise calculations, since no fitting functions would be present on the dummy
atoms and this can lead to large errors.
For examples and discussions of these aspects see Refs. [21-23]
28.10
Density-fitted LMP2 (DF-LMP2) and coupled cluster (DF-LCCSD(T0))
Density-fitting LMP2 and LCCSD calculations can be performed by adding the prefix DF- to
the command name. The input is as follows:
DF-LMP2,[options]
DF-LCCSD(T),[options]
Options for density fitting can be mixed with any options for LOCAL. Options for density fitting
can also be given on a DFIT directive (see section 11).
The most important options for density fitting in local methods are
BASIS MP2=string
Fitting basis set used in LMP2 and in LCCSD for integrals with up
to 2 external orbitals. If a correlation consistent basis set is used (e.g.
cc-pVTZ) the corresponding fitting basis for MP2 us used by default
(cc-pVTZ/MP2FIT). Otherwise the fitting basis set must be defined in
a preceding basis block (see section 13).
BASIS CCSD=string
Fitting basis set used in LCCSD for integrals over 3- and 4-external
orbitals. The default is BASIS MP2 and this is usually sufficient.
However, the accurate approximation of 4-external integrals in LCCSD
requires larger fitting basis sets than LMP2. Therefore, in order to
minimize fitting errors, it is recommended to use the next larger fitting basis, e.g., BASIS CCSD=VQZ for orbital basis VTZ.
LOCFIT=value:
If LOCFIT=1 local fitting is enabled. This is necessary to achieve
linear scaling in DF-LMP2 (see Refs. [11-14]). The errors introduced
by local fitting are usually very small, but there are some exceptions.
For instance, LOCFIT=1 must not be used in counterpoise calculations, see section 28.9.8) Note that for small molecules LOCFIT=1
can be more expensive than LOCFIT=0.
For further details and options for density fitting see section 11.