The electronic structure of the F2, Cl2, Br2 molecules: the description of charge-shift bonding within the generalized valence bond ansatz

In this paper, we present an alternative picture for the electronic structure of dihalogen molecules and for the physical origin of the charge-shift bonding effect. Absolute energies, binding energies, quadrupole moments and harmonic frequencies are determined for a hierarchy of methods from Hartree-Fock (HF), many forms of generalized valence bond (GVB) wavefunctions to Multi-Reference-MP2. All valence electron pairs are explicitly correlated in the GVB wavefunctions. It is shown that HF charge densities for the fluorine molecule are extremely inaccurate. This fact causes the HF canonical orbital basis for this molecule to be inadequate in low order correlation treatments in spite of the fact that there are no near degeneracies at the equilibrium distance. The accurate description charge fluctuation lone pair repulsions are essential for a proper assessment of the fluorine molecule binding energy, bond distance and harmonic frequency. These properties are well described by lifting the perfect-pairing restriction in a full-valence orbital optimized GVB-RCI (restricted-configuration-interaction) wavefunction. The accurate calculation of electron-electron cusps is of lesser importance in the description of the electronic structure of the fluorine molecule than usually considered. An analysis of the lone pair GVB natural orbitals provides a clear-cut understanding on the differences between fluorine and the other dihalogen molecules. Within our model, we conclude that among the dihalogen molecules the charge-shift bonding concept is meaningful only for the fluorine molecule.