Correlation effects in the spectra of polyacenes

Abstract

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.