It consists of electrostatics (E elst ), exchange or Pauli repulsion (E exch ), induction (E ind ), and dispersion (E disp ). In XSAPT, the first three of these components (E elst + E exch + E ind ) are modeled using second-order SAPT based on Kohn-Sham orbitals, as detailed elsewhere. In particular, this means that E elst consists of the exact Coulomb interaction between isolated-monomer charge densities, including nuclei and electrons. The induction energy includes both polarization and charge transfer. These can be separated, 155-160 but we have not attempted to do so because E ind is small and qualitatively unimportant for the systems considered here.