32.9.8 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 32.8.3). 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 32.8.4). 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=`

as described in section 32.9.3

A final warning concerns local density fitting (see sections 32.10 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]

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molpro@molpro.net 2015-08-28