Parameters ξ and λ in Eq. ( ) serve to count orders in a double perturbation expansion with the direct product |ψ A 0 |ψ B 0 as a reference state. A caveat is that |ψ A 0 |ψ B 0 is not antisymmetric with respect to exchange of electrons between A and B, since |ψ A 0 and |ψ B 0 were computed in isolation. The "symmetry adaptation" in SAPT is really an antisymmetry adaptation in which A ↔ B exchange is incorporated perturbatively, via an antisymmetrizer ÂAB . In practice, this means that the perturbation is VAB ÂAB , albeit with some additional simplifications for evaluating matrix elements of ÂAB , most notably the "single-exchange" (or "S 2 ") approximation. The presence of ÂAB in the perturbation gives rise to certain exchange terms in this antisymmetrized form of Rayleigh-Schrödinger perturbation theory, in addition to the usual terms that arise in the so-called polarization expansion. The latter is the perturbative approximation to the A• • • B interaction energy that is based on VAB alone, disregarding ÂAB . That expansion is valid for the long-range forces when the isolatedmonomer densities ρ A (r) and ρ B (r) do not overlap and there is no need to enforce the Pauli Principle for A ↔ B exchange.