An extended Heisenberg model for conjugated hydrocarbons. II. Kekule basis
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The valence bond (VB) model was implemented at a semiquantitative level through the extended Heisenberg model Hamiltonian. It was demonstrated that the improved VB model can be solved with reasonable accuracy in a very small basis, making it applicable to rather large molecules. Test calculations show that for benzoid hydrocarbons the Kekul basis can give a good semiquantitative description of ground-state energies, reaction energies, and singlet excitation energies. As an illustration of the use of the model for a chemical system whose size makes most computations beyond Hckel theory challenging, the ground and first excited state energies were computed for C60 buckminsterfullerene.