Noncovalent interactions are ubiquitous in nature and drive important chemical processes including crystal packing, 1-5 protein folding, and host-guest binding in pharmaceuticals, and are important to soft materials of interest in materials science applications. Despite their prevalence, noncovalent forces are often misunderstood by chemists. The framework of symmetryadapted perturbation theory (SAPT) offers an accurate and systematic ab initio approach to noncovalent interaction energies, including an energy decomposition into physically-meaningful components: electrostatics, Pauli repulsion, induction, and dispersion. This decomposition is useful for obtaining physical insight that is backed by reliable calculations. The SAPT energy decomposition is inherently better separable than methods based on supramolecular density functional theory (DFT) and can be used to develop physically meaningful force fields. The most widely used variant of SAPT, known as SAPT0, combines Hartree-Fock (HF) wave functions for the isolated monomers (as zeroth-order states) with second-order perturbation theory for the intermolecular Coulomb potentials. When used in conjunction with Kohn-Sham (KS) density functionals with correct asymptotic behavior, intramolecular electron correlation can be incorporated by substituting the KS determinant for the HF one, in a method that we have called SAPT0(KS). The use of asymptotically-correct functionals is crucial and the SAPT0 formalism should not be * herbert@chemistry.ohio-state.edu used with arbitrary density functionals. With that caveat, both traditional SAPT0 and SAPT0(KS) afford semiquantitative results at O(N 5 ) cost. The accuracy of both methods is limited by the accuracy of the dispersion interactions, which are not quantitative within the "uncoupled" second-order approximation that characterizes SAPT0, and which is similar to secondorder Møller-Plesset perturbation theory (MP2). To obtain quantitative accuracy, second-order dispersion must be replaced by better-performing alternatives. This has led to development of MP2 variants including MP2C and MP2D, as well as DFT-SAPT. These methods are much more accurate than SAPT0 but retain that method's O(N 5 ) cost. For MP2C and DFT-SAPT, a density fitting approximation is required in order to obtain fifth-order scaling. The "extended" (X)SAPT approach follows a somewhat similar strategy, replacing second-order dispersion with either ab initio dispersion potentials (XSAPT + ai D) or else with a many-body dispersion model (XSAPT + MBD). The latter approach is currently the best-performing variant of XSAPT, and is the one used herein. Unlike any of the aforementioned methods, XSAPT incurs only O(N 3 ) cost, and it can furthermore be extended to clusters of molecules using a self-consistent charge embedding to capture nonadditive polarization effects. For large systems, the monomer-based nature of XSAPT calculations makes this approach more affordable even than supramolecular DFT. Some limited basis-set testing of XSAPT methods has been reported in previous work, but only for total interaction energies. Herein, we also examine convergence of individual energy components, which allows us to consider whether earlier tests may have benefited from error cancellation amongst energy components that might exhibit different convergence behavior. The basisset dependence of traditional SAPT has also been carefully evaluated, but the especially slow convergence of the dispersion energy in the traditional approach means that those tests are not directly applicable to XSAPT. Moreover, because XSAPT was designed for large systems, we want to test Pople and Karlsruhe basis sets that are commonly used for large systems, whereas smallmolecule SAPT calculations have generally been limited to Dunning correlation-consistent basis sets. These tests allow us to consider whether hybrid basis-set combinations (in which different energy components are evaluated in different basis sets) might further improve the efficacy of XSAPT calculations. Such hybrid approaches take advantage of the inherent separability of the (X)SAPT interaction energy