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.