Ground-state properties of benzenoid hydrocarbons by simple bond orbital resonance theory approach

pi-electron energies and bond orders of benzenoid hydrocarbons with up to f ive fused hexagons have been considered by the simple Bond Orbital Resonanc e Theory (BORT) approach. The corresponding ground states were determined a ccording to four BORT models. In the first three models a diagonalisation o f the Huckel-type Hamiltonian was performed in the bases of Kekule of Kekul e and mono-Claus and of Kekule and Claus resonance structures, respectively . In the fourth model a simple PORT ansatz was used. According to this ansa tz, the ground state is a linear combination of the positive Kekule structu res, all with equal coefficients. It was shown that pi-electron energies an d bond orders obtained by these models correlate much better with the PPP e nergies and bond orders than with the Huckel energies and bond orders. This indicates that a simple BORT approach is quite reliable in predicting the more sophisticated PPP results. Concerning the relative performance of the four BORT models, the best results were obtained with the BORT ansatz. The performance deteriorates with the expansion of the basis set. This is attri buted to the fact that in these models the improvement of the basis set is not accompanied with the corresponding improvement of the Hamiltonian. Comp aring the BORT-ansatz bond orders with the Pauling bond orders, it was show n that BORT-ansatz bond orders correlate much better with the PPP bond orde rs.