An efficient linear-scaling CCSD(T) method based on local natural orbitals.

An improved version of our general-order local coupled-cluster (CC) approach [Z. Rolik and M. Kallay, J. Chem. Phys. 135, 104111 (2011)]10.1063/1.3632085 and its efficient implementation at the CC singles and doubles with perturbative triples [CCSD(T)] level is presented. The method combines the cluster-in-molecule approach of Li and co-workers [J. Chem. Phys. 131, 114109 (2009)]10.1063/1.3218842 with frozen natural orbital (NO) techniques. To break down the unfavorable fifth-power scaling of our original approach a two-level domain construction algorithm has been developed. First, an extended domain of localized molecular orbitals (LMOs) is assembled based on the spatial distance of the orbitals. The necessary integrals are evaluated and transformed in these domains invoking the density fitting approximation. In the second step, for each occupied LMO of the extended domain a local subspace of occupied and virtual orbitals is constructed including approximate second-order Moller–Plesset NOs. The CC equati...