Using the extended Thomas-Fermi version of density-functional theory (DFT), calculations are
presented for the barrier for the reaction Na_(20)^+ + Na_(20)^+ -> Na_(40)^(2+) The deviation from the simple
Coulomb barrier is shown to be proportional to the electron density at the bond midpoint of the supermolecule
(Na_(20)^+)_2. An extension of conventional quantum-chemical studies of homonuclear diatomic
molecular ions is then effected to apply to the supermolecular ions of the alkali metals. This then allows
the Na results to be utilized to make semiquantitative predictions of position and height of the maximum
of the fusion barrier for other alkali clusters. These predictions are confirmed by means of similar DFT
calculations for the K clusters.
