For noncovalent interactions, it is generally assumed that CCSD(T) is nearly the exact solution within the 1-particle basis set. For the S66 noncovalent interactions benchmark, we present for the majority of species CCSDT and CCSDT(Q) corrections with a polarized double-zeta basis set. For hydrogen bonds, pure London complexes, and mixed-influence complexes, CCSD(T) benefits from error cancellation between (usually repulsive) higher-order triples, T3−(T), and (almost universally attractive) connected quadruples, (Q). For π-stacking complexes, this cancellation starts breaking down and CCSD(T) overbinds; CCSD(T)Λ corrects the problem at the expense of London complexes. A fairly simple two-parameter model predicts CCSDT(Q)--CCSD(T) differences to 0.01 kcal/mol RMS, requiring no calculations that scale more steeply than O(N7).
@article{arxiv.2411.12004,
title = {Post-CCSD(T) corrections in the S66 noncovalent interactions benchmark},
author = {Emmanouil Semidalas and A. Daniel Boese and Jan M. L. Martin},
journal= {arXiv preprint arXiv:2411.12004},
year = {2025}
}