Accuracy of Explicitly Correlated Local PNO-CCSD(T).

Abstract

In recent years PNO-based local correlation methods have gained popularity since they allow Coupled Cluster (CC) calculations with reduced computational costs, yet only a few systematic studies concerning their accuracy are available, in particular for the explicitly correlated versions. In this work we take a deeper look at the explicitly correlated local PNO-CCSD(F12*)(T0) and PNO-CCSD(F12*)(T) methods. The first variant uses the so-called semicanonical triples correction (T0) which neglects off-diagonal elements in the occupied block of the Fock matrix. In PNO-CCSD(F12*)(T) this approximation is avoided by means of Laplace transformation techniques and convergence to the canonical results in the limit of no PNO truncation is restored. We assess the accuracy of both methods using well established benchmark sets for reaction energies and weak molecular interactions and take a look at a system with strong cooperative many-body effects. For reaction energies a close agreement with canonical methods is observed, and chemical accuracy can be reached. Also for weak intermolecular interactions the accuracy is easily controlled, and the methods even allow for improving existing benchmark data.