The *strong*, *close*, *weak* and *distant* pairs are selected using
distance or connectivity criteria as described in more detail in section 17.5.1.
*Strong* pairs are treated by CCSD, all other pairs by LMP2. However, if option `keepcls=1`
is ised, the LMP2 close pair amplitudes are included in the LCCSD amplitude equations for
the strong pairs. This is recommended (and default) for OSV and F12 calculations.
In triples calculations,
all orbital triples are included for which , , and are
*close pairs*. In addition, one of these pairs is restricted to be strong. The triples
energy depends on the strong and close pair amplitudes.
Thus, increasing the distance or connectivity criteria
for close and weak pairs will lead to more accurate triples energies (and also to more accurate
LCCSD energies if `keepcsl=1` is used). While for near
equilibrium properties like geometries and harmonic vibrational frequencies the default
values are normally appropriate, larger distance criteria are sometimes needed when
computing energy differences, in particular barrier heights. In cases of doubt,
`RWEAK` should first be increased until convergence is reached, and then
`RCLOSE` can be varied as well. Such tests can be performed with small basis
sets like cc-pVDZ, and the optimized values then be used in the final calculations with
large basis sets.

Pair approximations only affect the LCCSD calculations (LMP2 is only affected by `verydist`).
The defaults for `IWEAK`, `ICLOSE`, and `KEEPCLS` are wck=2,1,0, respectively, for PAO-LCCSD,
and 3,2,1 for OSV-LCCSD and all F12 methods.

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molpro@molpro.net 2018-08-16