of Kohn-Sham theory must use some form of approximation for the exchange-correlation energy V xc . Even so, in order to construct a molecular mechanics model, the majority of the non-classical components, such as the kinetic energy associated with electrons and their exchange-correlational energies, will also need to be approximated through empirical terms in an MM formulation. For simplicity, we combine both the kinetic contributions and those from V xc into a single functional K[ρ] ≡ T S [ρ] + V xc . Although this would be anything but practical if we were attempting to formulate approximations for a real Kohn-Sham system in which electron orbitals and their densities were solved from first principles, this will suffice for the purpose of formulating an approximate MM model.