Efficient Explicitly Correlated Coupled-Cluster Approximations

Explicitly correlated MP2-F12 and CCSD(T)-F12 methods are reviewed. We focus on the CCSD(T)-F12x (x = a,b) approximations, which are only slightly more expensive than their non-F12 counterparts. Furthermore, local approximations in the LMP2-F12 and LCCSD-F12 methods are described, which make it possible to treat larger molecules than with standard coupled-cluster methods. We demonstrate the practicability of F12 methods by large benchmark calculations for various properties, including reaction energies, vibrational frequencies, and intermolecular interactions. In these calculations, the newly developed VnZ-F12 orbital and OPTRI auxiliary basis sets by Peterson et al. are compared to other previously used basis sets. The accuracy and efficiency of local approximations is demonstrated for reactions of large molecules.