The domain-based local pair natural orbital (PNO) coupled-cluster
DLPNO-CCSD(T) method has been proven to provide accurate single-point
energies at a fraction of the cost of canonical CCSD(T) calculations.
However, the desired “chemical accuracy” can only be
obtained with a large PNO space and extended basis set. We present
a simple yet accurate and efficient correction scheme based on a perturbative
approach. Here, in addition to DLPNO-CCSD(T) energy, one calculates
DLPNO-MP2 correlation energy with the same settings as in the preceding
coupled-cluster calculation. In the next step, the canonical MP2 correlation
energy is obtained in the same orbital basis. This can be efficiently
performed for essentially all molecule sizes accessible with the DLPNO-CCSD(T)
method. By taking the difference between the canonical MP2 and DLPNO-MP2
energies, we obtain a correction term that can be added to the DLPNO-CCSD(T)
correlation energy. This way, one can obtain the total correlation
energy close to the limit of the complete PNO space (cPNO). The presented
approach allows us to significantly increase the accuracy of the DLPNO-CCSD(T)
method for both closed- and open-shell systems. The latter are known
to be especially challenging for locally correlated methods. Unlike
the previously developed PNO extrapolation procedure by Altun, Neese,
and Bistoni (J. Chem. Theory
Comput.2020,16,6142−614932897712), this strategy allows us
to get the DLPNO-CCSD(T) correlation energy at the cPNO limit in a
cost-efficient way, resulting in a minimal overall increase in calculation
time as compared to the uncorrected method.