The F2 Radical Anion

As is the case for its neutral homologue, the difluorine radical anion is a difficult test case for the calculation of its bonding energy. At the Hartree-Fock level, the bonding energy is about 4 kcal/mol, depending on whether the ROHF or UHF method is used. The experimental bond energy is 30 kcal/mol. 

In contrast, the MP2 and MP4 methods of theory are successful, and this success emphasizes the dynamic nature of electron correlation for this molecule. 

The computed equilibrium distance and bonding energy of F2- are displayed in Table 5. To appreciate better the sensitivity of active vs inactive orbitals to the breathing orbital effect, the latter has been introduced by steps In the first step no breathing orbitals are used (La = Lr, Ra = Rr, (Pi = cpf) this VBSCF calculation is nearly equivalent to the ROHF level. In the second step, only active orbitals are included in the breathing set (La 1- Lr, Ra Rr), while in the next step full breathing is permitted (La Lr, Ra Rr, cpi cpi ). The latter wave function, at the L-BOVB level, can be represented as in 26, 27 below. 

The breathing orbital effect, restricted to the active orbitals that are directly involved in the three-electron bond, already improves the bonding energy by some 17 kcal/mol relative to the ROHF value (Table 5). Extension of the 

Calculated equilibrium distances and dissociation energies for the 2 radical anion, 

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The importance and physical nature of dynamic correlation is even better appreciated in the case of 3e bonds, a type of bond in which the electron correlation is entirely dynamic, since there is no left-right correlation associated with odd-electron bonds. As noted earlier, the Hartree Fock and simple VB functions for 3e bonds (hence, GVB, SC, or VBSCF) are nearly equivalent and yield about similar bonding energies. Taking the F2 radical anion as an example, it turns out that, compared to the experimental bonding energy of... 

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