Thus far we have shown that ∆E AB can be written as a functional of the pair (ρ A , ρ B ). We try to provide an expression for it via a heuristic approach where we separate ∆E XC AB into electron-electron and electron-nuclei interaction components. In each of the terms we use a K th order approximation requiring the fitting of K parameters with respect to reliable experimental data, accurate (yet computationally demanding) ab initio calculations or a combination of both. The parameters are the weights of different functionals of the electronic densities of the non-interacting molecules. ∆E XC AB is then defined as: