This section explores the use of XSAPT + MBD as a tool for understanding the physics of intermolecular interactions, taking advantage of the dimer EDA in Eq. ( ) and relying on the sub-kcal/mol accuracy that was established in Section 2.1 and elsewhere. We first establish some basics about short-range repulsion, which originates in the Pauli Principle and not with Coulomb interactions, as discussed in Section 3.1. Then, the MBD dispersion model is compared directly to dispersion energies obtained from a popular DFT-based EDA scheme, 13 in Section 3.2. That discussion highlights the fact that E MBD+esDQ disp in Eq. ( ) agrees very well with SAPT2+(3) dispersion energies, even though it was not parameterized using these data (unlike the ai D models). That discussion also highlights ambiguities in the definition of E disp in the DFT-based EDA. Having established the veracity of the XSAPT + MBD energy decomposition, we apply it to analyze π-π interactions in Section 3.3. Finally, hydrogen bonding in F -(H 2 O) is examined in Section 3.4, using the SAPT-cDFT decomposition to isolate E CT and thereby elucidate the role of CT in then anion-water hydrogen bond.