NATURE OF THE DIFFERENTIAL ELECTRON CORRELATION IN 3-ELECTRON BOND DISSOCIATIONS - EFFICIENCY OF A SIMPLE 2-CONFIGURATION VALENCE-BOND METHOD WITH BREATHING ORBITALS

The recently proposed breathing orbital valence bond method is applied to the study of three-electron bonds. It consists of describing an electronic system with a minimal number of configurations, one per possible Lewis structure, each configuration having its specific set of orbitals. The method is tested at various levels of approximation on the F-2(-) radical anion and reproduces a satisfactory bonding energy with a wave function involving only two determinants. The nature of the differential electron correlation in post-Hartree-Fock calculations is analyzed and shown to correspond to an intantaneous adaptation of the orbitals to the charge fluctuation that occurs in the three-electron bond. The method is further applied to the (H3N-NH3)(+) and Cl-2(-) three-electron bonds. Some general tendencies are derived, stating that the differential correlation energy should gradually decrease as the bonded atoms are taken from right to left or from top to bottom of the periodic table.