In this part, we will consider the approximation to ∆X by employing the single Slater determinant wavefunction, corresponding to the same level approximation as in SAPT0. Such an approximation can smoothly be changed to the Kohn-Sham (KS) description, analogously to the SAPT(DFT) method. For convenience, let us use the same notation ϕ A,B for the HF (or KS) Slater determinant as we used in the previous section. It is also helpful to introduce the normal-ordered form of spinorbital replacement operators a κ λ = a † κ a λ , and