When the linear bond-energy/bond-order relationship is applied to a diatomic molecule, a fundamental deficiency becomes obvious. As nucleus 1 penetrates the exponentially decaying electron density around atom 2, the electron 2-nucleus 1 attraction is reduced and is no longer sufficient to cancel the nuclear-nuclear repulsion completely. Lennard-Jones proposed that short-range repulsion could be expressed conveniently as a power law of the form 1/R 12 , but London showed that an exponential form is more appropriate. Several exponential forms have been proposed. A simple exponential form is employed in the BEBOP model: