29.9.6 Pair Classes

The strong, close, weak and distant pairs are selected using distance or connectivity criteria as described in more detail in section 29.7. Strong pairs are treated by CCSD, all other pairs by LMP2. In triples calculations, all orbital triples $(ijk)$ are included for which $(ij)$, $(ik)$, and $(jk)$ 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. The close pair amplitudes are taken from the LMP2 calculation. Thus, increasing the distance or connectivity criteria for close and weak pairs will lead to more accurate triples energies. 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.