Hartree-Fock instabilities and electronic properties

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