TCB Publications - Abstract

Paul Tavan and Klaus Schulten. The low-lying electronic excitations in long polyenes: A PPP-MRD-CI study. Journal of Chemical Physics, 85:6602-6609, 1986.

TAVA86A A correct description of the electronic excitations in polyenes demands that electron correlation is accounted for correctly. Very large expansions are necessary including many electron configurations with at least one, two, three, and four electrons promoted from the Hartree-Fock ground state. The enormous size of such expansions had prohibited accurate computations of the spectra for polyenes with more than ten $\pi$ electrons. We present a multireference double excitation configuration interaction method (MRD-CI) which allows such computations for polyenes with up to $16\pi$ electrons. We employ a Pariser-Parr-Pople (PPP) model Hamiltonian. For short polyenes with up to ten electrons our calculations reproduce the excitation energies resulting from full-CI calculations. We extend our calculations to study the low-lying electronic excitations of the longer polyenes, in particular, the gap between the first optically forbidden and the first optically allowed excited singlet state. The size of this gap is shown to depend strongly on the degree of bond alternation and on the dielectric shielding of the Coulomb repulsion between the $\pi$ electrons.

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