| Abstract: | Single-zeta and π-electron double-zeta basis sets are used to examine some theories of the origin of the stability of the D2h isomer of O4, using ab initio valence-bond procedures. With these basis sets, resonance between covalent-type (i.e., O2 ·· O2) valence-bond structures does not lead to a stabilization of the dimer relative to the separated monomers. When basis sets of the same size are used to construct wave functions for covalent and ionic structures, covalent-ionic resonance (i.e., O2 ·· O2 ↔ O2+ ·· O2− ↔ O2− ·· O2+) is also unable to stabilize the dimer. Without consideration of the basis-set superposition error, stability is obtained when the size of the AO basis is increased for the dimer relative to the monomer, either via the basis for the ionic structures or by the inclusion of midbond functions. Brief consideration is given to an increased-valence structure for the dimer. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 547–555, 1997 |
