INTERACTION OF S2 PAIRS IN BE-2 AND C-2 - THE UHF INSTABILITY, SYMPTOM OF AN ATOMIC PROMOTION

The correct treatment of Be2 bonding has required large basis sets and very extensive CIs when starting from symmetry-adapted MOs. The nature of this short weak bond remains unclear, but the bound UHF solution leads to a picture in terms of a resonance between {A figure is presented} and {A figure is presented} determinants, i.e. to an interaction between 3P atoms. Similarly for C2 the lowest UHF solution shows that at short interatomic distance the four σ electrons are spread into four open shells, the leading VB configuration being a product of 5S carbon atoms {A figure is presented}. These examples illustrate the ability of symmetry-broken HF solutions to reveal clearly the main physical trends introduced by electronic correlation and to serve as a link between the classical MO and VB languages. © 1990.