Optimal parameterisation of the Pariser-Parr-Pople model for benzene, bi-phenyl and trans-stilbene.

By means of exact diagonalisation studies of transition energies and oscillator strengths, we obtain an appropriate parameterisation of the Pariser-Parr-Pople hamiltonian for pi-conjugated systems containing phenyl units, such as Poly-(para-Phenylene), Poly- (para-Phenylene-Vinylene) and their short chain oligomers. We do this by adjusting the parameters in the model to obtain an optimal fit to the experimental values of the low lying excitations of benzene, bi-phenyl and trans-stilbene. We use various trial combinations of terms - including, for example, second neighbour hopping and variable site energies - our aim being to produce a reliable parameterisation involving a minimal number of terms, thus enabling efficient porting of the parameterisation into more complex calculations, such as DMRG calculations for long chain materials and calculations on substituted systems.