As an alternative, XSAPT uses a monomer basis to compute the SCF wavefunctions, ensuring that the total SCF cost is O(N ) with respect to the number of monomers. As a means to capture some of the intermolecular CT that is lost in the monomer-centered approach, XSAPT instead employs a "projected" (pseudocanonicalized) monomer basis to evaluate the dimer SAPT corrections. For the dimer AB, this means first obtaining density matrices P A and P B from monomer-centered SCF calculations, then building monomer Fock matrices F A and F B using the block-diagonal dimer density matrix P A ⊕ P B . Each of these Fock matrices is then transformed into a pseudocanonical basis that diagonalizes the occupied-occupied and virtual-virtual blocks independently, as this does not alter the monomer densities that are invariant to such transformations. We omit the first-order (or "non-Brillouin singles") correction, which is formally introduces into the perturbation expansion by this transformation. As compared to a proper dimer basis, the use of this pseudocanonicalized basis results in acceptable errors in conventional pairwise SAPT0 calculations, e.g., a 10% error in the total interaction energy for (H 2 O) 2 . 32 For a system of more than two monomers, the aforementioned procedure is repeated for all pairs AB, and SAPT calculations are evaluated in a pairwiseadditive manner as described in Section 1.3.3. As indicated already in Eq. ( ), XSAPT is not a single model but rather a family of methods, 37,42 or a platform for accurate calculation of intermolecular interactions that is also equipped with an energy decomposition scheme. By its nature, this requires some decisions on the user's part and makes XSAPT less straightforward than a typical DFT-based EDA. The latter methods only require the user to select an XC functional and basis set, push a button, and accept the results at face value. That final facet cannot be overemphasized, although it is overlooked by most users. XSAPT allows a user to test the impact of different approximations, with the goal of obtaining high-quality interaction energies and well-defined energy components or in other words, answers that are physically understandable. (In principle, the DFT-based approach could be tested with a variety of functionals but in our experience, few users do this.) In our view, the added complexity of XSAPT is a virtue and not a drawback, and it is one that is shared by the more conventional and systematically improvable SAPT formalism. With conventional SAPT, however, the levels of approximation subsequent to SAPT0 come with prohibitive cost. XSAPT can be understood as an attempt to match those approximations while retaining O(N 3 ) complexity, which is necessary to tackle large systems.