Valence Bond Methods Based on Semilocalized Orbitals

2 VALENCE BOND METHODS BASED ON SEMILOCALIZED ORBITALS 

All the VB methods that deal with semilocalized orbitals use a generalization of the Coulson—Fischer idea (12), whereby a bond is described as a singlet coupling between two electrons in nonorthogonal orbitals that possess small delocalization tails resulting from the variational orbital optimization. Albeit formally covalent, this description implicitly involves some optimal contributions of ionic terms, as a decomposition of the wave function in terms of pure AO determinants would show (see Eqs. 3.5 and 3.6). For a polyatomic 

In a more restrictive definition of the CF orbitals, each orbital is allowed to delocalize only onto the atom to which it is bonded in the VB structure under consideration such orbitals are called bond-distorted orbitals (BDOs). 

The BDOs have the advantage of allowing an unambiguous correspondence between the mathematical expression of a VB structure and the associated Lewis structure. On the other hand, owing to the delocalization tails in these orbitals, neither OEOs nor BDOs allow the distinction to be made between the 

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