MOLPRO

29 LOCAL CORRELATION TREATMENTS 223
We recommend in such cases to add an ACCU,14 directive (possibly even ACCU,16) after the
HF command. Indicative of insufficient SCF accuracy are small positive energy changes near
the end of the geometry optimization.
29.9.8 Intermolecular interactions
Local methods are particularly useful for the calculation of weak intermolecular interactions
since the basis set superposition error (BSSE) is largely reduced [1,13,14] and counterpoise corrections
are usually not necessary (provided the BSSE of the underlying Hartree-Fock is small).
However, one must be careful to define the domains properly and to include all intermolecular
pairs at the highest computational level. A convenient way to define appropriate domains and
pair lists is to use the option INTERACT=1. If this option is given, individual molecules are
identified automatically and all intermolecular pairs are automatically treated as strong pairs and
included in the LCCSD. Similarly, appropriate triples lists are generated for LCCSD(T) calculations.
It is required that all orbital domains are located on individual molecules. Note however
that the inclusion of the intermolecular pairs strongly increases the number of strong pairs and
triples, and therefore high-level calculations can become very expensive.
For calculations of interaction potentials of weakly interacting systems, the domains of the subsystems
should be determined at a very large distance and saved using the SAVE=record option
on the LOCAL or MULTP directive, or the SAVE directive (see section 29.8.3). If the asymptotic
energy is not needed it is sufficient to do this initial calculation using option DOMONLY=1).
These domains should then be reused in the subsequent calculations at all other intermolecular
distances by using the START=record option or the START directive (see section 29.8.4). Only
in this way the basis set superposition error is minimized and normally negligible (of course,
this does not affect the BSSE for the SCF, and therefore the basis set should be sufficiently large
to make the SCF BSSE negligible).
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 29.9.3
A final warning concerns local density fitting (see sections 29.10 and 15): 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]
29.10 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 15).
The most important options for density fitting in local methods are