where β 12 is now independent of R 12 . The first term in Eq.10 represents the covalent bond energy as a function of bond order (BEBO). The second term represents charge transfer between atoms 1 and 2 as expressed through the atomic populations. The Coulomb attraction between the charges induced by this charge transfer tends to cancel the charge transfer energy, except in the case of ion such as Li + F -. If we set: