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32.10 Density-fitted LMP2 (`DF-LMP2`), LDCSD (`DF-LDCSD`),
and coupled cluster (`DF-LCCSD(T0)`)

Density-fitting LMP2, LDCSD 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 15).

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 11).
`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 32.9.8)
Note that for small molecules `LOCFIT=1` can be more expensive than
`LOCFIT=0`.

For further details and options for density fitting see section 15.

molpro@molpro.net 2019-03-19