Pursuing the basis set limit of CCSD(T) non-covalent interaction energies for medium-sized complexes: case study on the S66 compilation

Our recently developed conventional, explicitly correlated, and local natural orbital (LNO) based coupled cluster with single, double, and perturbative triple excitations [CCSD(T)] methods are utilised to closely approach the basis set limit of interaction energies between medium-sized molecular complexes. The study is partly motivated by our previous finding that state-of-the-art diffusion Monte Carlo and CCSD(T) interaction energies are not in complete agreement even for some of the dimers in the popular S66 compilation [Al-Hamdani and Nagy et al. Nat. Commun. (2021) 12:3927]. We improve upon the previous S66 CCSD(T) reference by using up to the quadruple-<i>ζ</i> (QZ) level CCSD(T) contributions, utilising our recent (T+) approach and suggesting a more robust explicitly correlated CCSD reference. Our efficient LNO-CCSD(T) method allowed us to also compute tightly converged, quintuple-<i>ζ</i> level interaction energies for the S66 set. Moreover, the new CCSD(T)/QZ references enabled the so far most rigorous benchmark of the LNO and other popular local correlation approaches on the S66 set. The accuracy of local approximations was found in the few 10 to few 100 cal/mol interaction energy error range, assuming sufficiently tight settings, such as the Tight and the Normal settings of LNO-CCSD(T) threshold combinations, respectively.