Since calculating exact electronic wavefunctions for many-electron systems analytically is impossible. While the full configuration-interaction (FCI) method can generate an exact solution within a finite basis set of algebraic approximation, it is prohibitively costly due to its factorial complexity. Consequently, Ψ A and Ψ B are often substituted by Hartree-Fock determinants as a starting approximation. This treatment obviously results in the neglect of electron correlation effects within the monomers. However, intramonomer correlation can be built-in through additional perturbations. Constructing a perturbation expansion where the sum ĤA + ĤB is taken as a zeroth-order