Approaching the Basis Set Limit of CCSD(T) Energies for Large Molecules with Local Natural Orbital Coupled-Cluster Methods.

Recent optimization efforts and extensive benchmark applications are presented illustrating the accuracy and efficiency of the linear-scaling local natural orbital (LNO) coupled-cluster single-, double-, and perturbative triple-excitations [CCSD(T)] method. A composite threshold combination hierarchy (Loose, Normal, Tight, etc.) is introduced, which enables black box convergence tests and is useful to estimate the accuracy of the LNO-CCSD(T) energies with respect to CCSD(T). We also demonstrate that the complete basis set limit (CBS) of LNO-CCSD(T) energies can be reliably approached via basis set extrapolation using large basis sets including diffuse functions. Where reference CCSD(T) results are available, the mean (maximum) absolute errors of the LNO-CCSD(T) reaction and intermolecular interaction energies with the default Normal threshold combination are below 0.2-0.3 (0.6-1.0) kcal/mol, while the same measures with the Tight setting are 0.1 (0.2-0.5) kcal/mol for all the tested systems including highly complicated cases. The performance of LNO-CCSD(T) is also compared with that of other popular local CCSD(T) schemes. The exceptionally low hardware requirements of the present scheme enables the routine calculation of benchmark-quality energy differences within chemical accuracy of CCSD(T)/CBS for systems including a few hundred atoms. LNO-CCSD(T)/CBS calculations can also be performed for more than 1000 atoms with 45,000 atomic orbitals using a single, six-core CPU, about 100 GB memory, and comparable disk space.