A two-state (2-MO) model for the low-lying long axis-polarised excitations
of poly(p-phenylene) oligomers and polymers is developed. First we derive s
uch a model from the underlying Pariser-Parr-Pople (P-P-P) model of pi-conj
ugated systems. The two states retained per unit cell are the Wannier funct
ions associated with the valence and conduction bands. By a comparison of t
he predictions of this model to a four-state model (which includes the non-
bonding states) and a full P-P-P model calculation on benzene and biphenyl,
it is shown quantitatively how the 2-MO model fails to predict the correct
excitation energies. The 2-MO model is then solved for oligophenylenes of
up to 15 repeat units using the density matrix renormalisation group (DMRG)
method. It is shown that the predicted lowest-lying dipole-allowed excitat
ion is ca. 1 eV higher than the experimental result. The failure of the 2-M
O model is a consequence of the fact that the original HOMO and LUMO single
particle basis does not provide an adequate representation for the many-bo
dy processes of the electronic system. (C) 1999 Elsevier Science B.V. All r
ights reserved.