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 oscill ator strengths, we obtain an appropriate parameterisation of the Pariser-Pa rr-Pople hamiltonian for pi-conjugated systems containing phenyl units, suc h as Poly-(para-Phenylene), Poly- (para-Phenylene-Vinylene) and their short chain oligomers. We do this by adjusting the parameters in the model to ob tain an optimal fit to the experimental values of the low lying excitations of benzene, bi-phenyl and trans-stilbene. We use various trial combination s of terms - including, for example, second neighbour hopping and variable site energies - our aim being to produce a reliable parameterisation involv ing a minimal number of terms, thus enabling efficient porting of the param eterisation into more complex calculations, such as DMRG calculations for l ong chain materials and calculations on substituted systems.