  In this paper we present large-scale correlated calculations of linear
optical absorption spectrum of oligo-acenes containing up to seven benzene
rings. For the calculations we used the Pariser-Parr-Pople (P-P-P) Hamiltonian,
along with the configuration interaction (CI) technique at various levels such
as the full CI (FCI), the quadruple CI (QCI) and multi-reference
singles-doubles CI (MRSDCI). The role of Coulomb parameters used in the P-P-P
Hamiltonian was examined by considering standard Ohno parameters, as well as a
screened set of parameters. A detailed analysis of the many-body character of
the important excited states contributing to the linear absorption has also
been performed. The results of our calculations have been compared extensively
with the theoretical work of other authors, as well as with the experiments.
