  In this paper, we perform large-scale electron-correlated calculations of
optoelectronic properties of rectangular graphene-like polycyclic aromatic
hydrocarbon molecules. Theoretical methodology employed in this work is based
upon Pariser-Parr-Pople (PPP) $\pi$-electron model Hamiltonian, which includes
long-range electron-electron interactions. Electron-correlation effects were
incorporated using multi-reference singles-doubles configuration-interaction
(MRSDCI) method, and the ground and excited state wave functions thus obtained
were employed to calculate the linear optical absorption spectra of these
molecules, within the electric-dipole approximation. As far as the ground state
wave functions of these molecules are concerned, we find that with the
increasing size, they develop a strong diradical open-shell character. Our
results on optical absorption spectra are in very good agreement with the
available experimental results, outlining the importance of
electron-correlation effects in accurate description of the excited states. In
addition to the optical gap, spin gap of each molecule was also computed using
the same methodology. Calculated spin gaps exhibit a decreasing trend with the
increasing sizes of the molecules, suggesting that the infinite graphene has a
vanishing spin gap.
