in Eq. ( ) are the first-order SAPT electrostatic and exchange energies, while E disp is the dispersion energy computed using the MBD model . The induction energy comes from second-order SAPT but warrants additional discussion and is detailed in Section 2.3. Previous basis-set testing of SAPT0 + MBD reveals that polarized triple-ζ basis sets are both necessary and sufficient to obtain converged energetics . This is a unique feature of our hybrid approach to SAPT , which replaces the very slow basis-set convergence of perturbative dispersion with a model (MBD) that converges quickly, with the density. Tests for Cl -(aq) in Fig. demonstrate that interaction energies computed using the 6-311+G(d,p) basis set agree with SAPT0 + MBD/def2-TZVPD values to within an average of 2.0 kcal/mol, in a total interaction energy that averages -106 kcal/mol. Relative to the More important than these relatively small differences is the fact that instantaneous values of E int fluctuate from snapshot to snapshot in a similar way in either basis set. For these calculations, which involve Cl -(H 2 O) n with an average of n = 28 water molecules, SAPT0 + MBD/6-311+G(d,p) calculations are 17× faster than the corresponding calculations with def2-TZVPD. (This speedup results largely from the absence of diffuse functions on hydrogen but also benefits from Q-Chem's very efficient handling of sp shells in Pople basis sets.) In the present work, we are concerned with comparisons between bulk and interfacial behavior rather than absolute interaction energies, and the need for ensemble averaging requires high throughput. As such, 6-311+G(d,p) is used for all subsequent SAPT calculations.