Abstract
The semi-empirical approximations of Pariser, Parr and Pople (PPP) are used in the valence bond (VB) method. Simple formulas are presented for the calculation of the diagonal and off-diagonal elements of the energy matrix based on the complete set of covalent and polar structures. The method has been applied to butadiene and benzene. In order to interpret the u.v. spectra of these molecules a very large number of polar structures have to be taken into account, whereas in the molecular orbital (MO) theory only a small number of excited configurations are needed. The main advantage of the VB method in comparison with the MO method, using in both cases the PPP approximations, is the simplicity of the expressions for the matrix elements between the VB structures. For the performance of complete calculations therefore the VB method is to be preferred.
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