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.