The formulation of our model starts with the use of atom centered densities and effective nuclei as opposed to atom centered point charges. In this formalism, each atomic site has an associated effective nuclei (Z eff = Z -2 for heavy atoms, Z eff = 1 for hydrogen, Z eff = 0 for lone-pairs) and an electron density ρ i (r) that sum to the total density distribution ρ tot (r),