Influence of Three-Body Effects on Halogen Bonding

We construct a new noncovalent benchmark dataset 3BXB that combines halogen-bonded bimolecular complexes from the SH250 dataset [Kriz and Rezac, Phys. Chem. Chem. Phys. 2022, 24, 14794] with a third interacting partner, either H₂O or CH₄. The reference total and three-body interaction energies are computed at the CCSD(T) level. To shed light on the physical origins of binding and cooperativity in complexes of this kind, several symmetry-adapted perturbation theory (SAPT)-based energy decompositions were performed for both pairwise additive and nonadditive terms. We found that the two-body attractions in the 3BXB complexes are dominated by either electrostatics or dispersion, while the three-body effect is dominated by induction and can be either attractive or repulsive. An accurate recovery of reference interaction energies is attained by the wavefunction-based two-body SAPT variants including the $δ$MP2 correction, combined with the SAPT(DFT) estimates of nonadditive induction and first-order exchange and any estimate of nonadditive dispersion. The values for the latter term are sometimes quite inconsistent between different approaches; fortunately, nonadditive dispersion is a relatively minor effect for complexes studied here, and all reasonable estimates lead to total interaction energies of similar accuracy.