Hartree-Fock instabilities and electronic properties

Hartree-Fock instabilities are investigated for about 80 compounds, from acetylene to mivazerol (27 atoms) and a cluster of 18 water molecules, within a double zeta basis set. For most conjugated systems, the restricted Hartree-Fock wave function of the singlet fundamental state presents an external or so-called triplet instability. This behavior is studied in relation with the electronic correlation, the vicinity of the triplet and singlet excited states, the electronic delocalization linked with resonance, the nature of eventual heteroatoms, and the size of the systems. The case of antiaromatic systems is different, because they may present a very large internal Hartree-Fock instability. Furthermore, the violation of Hund's rule, observed for these compounds, is put in relation with the fact that the high symmetry structure in its singlet state has no feature of a diradical-like species. It appears that the triplet Hartree-Fock instability is directly related with the spin properties of nonnull orbital angular momentum electronic systems. (C) 2000 John Wiley & Sons, Inc.