Ab initio potential energy curve of F2. IV. Transition from the covalent to the van der Waals region: competition between multipolar and correlation forces.

The potential energy curve of the fluorine molecule in the ground electronic state (1)Sigma(g)(+) is determined and analyzed in the long-range region. The analysis is based on expressing the potential as the sum of the potential energy curve of the uncorrelated, but properly dissociating wave function and the correlation energy contribution. It is shown that, in the long-range region, the former becomes identical with the interaction between the quadrupoles of the fluorine atoms and the latter becomes the London dispersion interaction. The former is repulsive because of the coaxial quadrupole alignments in the (1)Sigma(g)(+) ground state and proportional to 1/R(5). The latter is attractive and proportional to 1/R(6). There moreover exists an additional repulsive force due to the loss of spin-orbit coupling upon the bond formation. As a result of these antagonistic interactions, the potential energy curve has a barrier at about 4 A, with a value about +0.04 mhartree. The descent of the potential toward the minimum, when the atoms approach each other from infinity, begins therefore only at internuclear distances less than about twice the equilibrium distance and is then very steep.