Correlation effects in the spectra of polyacenes

In order to describe the electron correlation in the excited singlet π,π states of the polyacenes [C_4n+2H_2n+4], we have carried out PPP-SCF-CI calculations including all single and double excitations in the CI expansion up to n= 5, including all triple excitations up to n= 3, and all quadruple excitations up to n= 2. Compared to previous CI descriptions which included single excitations only, e.g., the classic work of Pariser [J. Chem. Phys. 24, 250 (1956)], our calculations lead us to predict the following: (1) "new" excited states entailing the promotion of two electrons from the ground state (some of them predicted previously by other authors), and (2) a partial reordering of those (well-known) excited states already accounted for by a S-CI representation. Single and double excitations in a CI expansion (D-CI) satisfactorily describe the ordering of all excited states up to 7 eV; the effect of higher excitations is to correct the excitation energies overestimated by the D-CI description. Our predicted spectra provide a consistent assignment of all one- and two-photon spectral data but do not yield a quantitative agreement.