providing a form of energy decomposition analysis (EDA). The components in Eq. ( ) include electrostatics (E elst ), exchange or Pauli repulsion (E exch ), dispersion (E disp ), and induction (E ind ), the latter of which contains both polarization and charge transfer. The elementary physics of these components is described elsewhere. For the present discussion, it suffices to note that E disp is cleanly separable from the remaining components, identifiable as a subset of the excitations in the perturbation series. This is different from the situation in methods such as DFT+D or DFT+MBD, where long-range dispersion is shouldered by the ad hoc correction but middle-range dispersion arises from the XC functional. The details of that separation depend sensitively on the length scale of the semilocal functional, which is difficult to assess a priori. In traditional SAPT, dispersion is defined in terms of simultaneous excitations on two individual monomers that create fluctuations in the charge density, leading to long-range multipolar interactions beyond those associated with any permanent multipole moments. Equivalently, these fluctuations engender transient changes in the frequency-dependent polarizability, and in either conceptual picture dispersion arises strictly from electron correlation. At second order in intermolecular perturbation theory, which defines a variant known as "SAPT0", the total dispersion energy is a sum of second-order dispersion and exchange-dispersion terms: 249