Ab initio valence bond calculations are performed for the three lowest
states of the oxygen molecule ((3) Sigma(g)(-), (1) Delta(g), and (1)
Sigma(g)(+)). One objective of the present study was to make a contri
bution to previous valence bond discussions about the oxygen ''double'
' bond. Further, we study the origin of a small barrier in the potenti
al energy surface of the ground state. Two compact models are employed
to maintain the clear picture that can be offered by the valence bond
method. The first model has only the Rumer structures that are essent
ial for bonding and a proper dissociation. The second model, in additi
on, has structures which represent excited atoms. These prove to be im
portant for the dissociation energies. For both models, the orbitals a
re fully optimized. The spectroscopic data obtained are significantly
better than are the (few) valence bond results on O-2 that have been p
ublished and have the quality of multiconfiguration self-consistent fi
eld calculations in which the same valence space is used. The ''hump''
in the potential energy surface of the ground state is shown to arise
from a spin recoupling. The free atoms correspond to a spin coupling
that is incapable of describing the formation of bonds. Only at short
distances, an alternative spin coupling provides bonding and the repul
sive curve is converted into an attractive one. Our results on this su
bject support a valence bond explanation previously given by McWeeny [
R. McWeeny, Int. J. Quantum Chem. Symp. 24, 733 (1990)]. (C) 1996 John
Wiley & Sons, Inc.