where 3 Computational Details 3.1. XSAPT Methods. All calculations were performed using Q-Chem v. 5. Integral and shell-pair thresholds were set to 10 -12 a.u., the SCF convergence threshold was set to 10 -7 Ha, and the SAPT and XSAPT calculations use the "projected" basis set. The latter is an alternative to the dimer basis set that is traditionally used to compute monomer wave functions for SAPT, but in clusters with more than two monomers this would entail a pairwise SCF cost. In the projectedbasis approach, each monomer's SCF wave function is computed in the monomer basis but then a pseudocanonicalized dimer basis is used for the subsequent pairwise SAPT calculations. Most calculations reported here are performed at the XSAPT + MBD level of theory but some tests will be reported using XSAPT + ai D3, where "ai D3" denotes the third-generation ab initio dispersion potential developed by Lao and Herbert. The performance of XSAPT + ai D3 is marginally better than that of the second-generation version (XSAPT + ai D2), with slightly better coverage of π-stacking in the training set used to parameterize ai D3. These methods outperform the first-generation version, which we do not recommend. 3.2. Density Functionals. For SAPT0(KS) and XSAPT calculations, the range separation parameter in the LRC-ωPBE functional must be tuned individually for each monomer. We do this at the LRC-ωPBE/ def2-TZVP level, then use that value in all subsequent calculations regardless of basis set. (The GDD-tuned values of ω, for each of the systems considered in this work, can be found in Tables S1-S7 of the Supporting information.) Previous work has shown that tuned values of ω are sensitive to the fraction of short-range exact exchange but for a given density functional, these values are rather insensitive to the choice of basis set. The SG-1 quadrature grid is used for LRC-ωPBE.