Valence bond theory coulomb energy

This section deals with analytical properties of the Hiickel molecular orbital theory and the associated isolated molecule method of predicting the active positions in a conjugated molecule. We shall deal with polarizability coefficients defined as certain partial derivatives with respect to the coulomb and resonance integrals described in Section III. The important derivatives are those relating to the total tt electron energy and to the charges q, fi ee valences and bond orders... 

The quantum mechanical and bond valence theories can thus be seen as complementary descriptions, both being exact representations of the Coulomb force, one expressed through the electric potential, the other through electric field. The quantum model gives the energy of a molecule or crystal and describes its excited states. It also gives the distribution of the electron density, but it does not give the... 

Clearly bond valence theory does not contain the whole story. It cannot be used to calculate energies or determine the distribution of the electrons that give rise to bonding. This information requires a quantum treatment using the Coulomb potential, but bond valence theory provides information that quantum theory cannot. It can be used to determine the atomic positions that are required before quantum calculations can be made. If the quantum approach is a physicist s description of the atom, the bond valence theory is the corresponding chemist s description of the bond. 

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