Wave functions model polyene systems

We outline the CASVB strategy, which may be used either to generate very compact modem valence bond representations of CASSCF wave functions or to optimize general types of modem VB wave function. Various aspects of the methodology are illustrated by means of applications to the ground state of benzene, to the X A and a A state of FeH, and to the two lowest Ag states of various model polyene systems. 

As we have indicated, the CASVB procedures may also be used for the fully-variational optimization of modem VB wave functions. This allows us to investigate for the ground states of these model polyene systems the extent to which the modem VB representations obtained from the CASSCF wave functions by means of the overlap criterion resemble the corresponding flilly-variational spin-coupled results. The energies from such flilly-variational calculations, which include full optimization of the core orbitals, are labelled SC in Table VIII. All of the CASVB wave functions were found to be symmetry pure, without the need to impose any symmetry relationships amongst the orbitals. 

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