Quantum-mechanical values of E int were computed using SAPT based on Hartree-Fock (HF) wave functions for the monomers and second-order perturbation theory for the intermolecular Coulomb operators, a method that is usually called SAPT0 , and which is closely related to second-order Møller-Plesset perturbation theory (MP2). However, because second-order dispersion is far from quantitative , we replace it in these calculations with a many-body dispersion (MBD) model , in what we have termed a "hybrid" or "extended" form of SAPT . This method will be designated as SAPT0 + MBD. At this level of theory, results for small-molecule data sets suggests that errors in E int are within ∼ 1 kcal/mol of the best-available benchmarks , provided that adequate basis sets are employed . All electronic structure calculations were performed using the Q-Chem software, v. 5.4 .