The short-range repulsion term in Eq.14 plays a significant role in maintaining the viability of the linear BEBOP relationship expressed in Eq.12. Without this short-range repulsion, we could not describe the wide variety of C∼C bonds (Figure ), unless we employed a totally unjustified nonlinear bond-energy/bond-order relationship. The physically necessary short-range-repulsion term is thus included for practical reasons, even if we do not attempt to explore potential energy surfaces. This is not the case for all atom pairs, AB. For example, we could describe C∼N bonds quite adequately without the short-range repulsion (Figure ). The optimum value for ζ CN is therefore very large (ζ CN = 13.84 Å -1 ), effectively removing any influence from the region around R e . The BEBOP model includes the attractive linear bond-energy/bond-order relationship in Eq.12, the short-range exponential repulsion in Eq.