"Electronic excitations in long polyenes revisited" Maximilian Schmidt and Paul Tavan
J. Chem. Phys. 136, 124309 (2012).
Abstract: We apply the valence shell model OM2 (Weber and Thiel, Theoret. Chem. Acc. 103, 495, 2000) combined with multireference configuration interaction (MRCI) to compute the vertical excitation energies and transition dipole moments of the lowenergy singlet excitations in the polyenes with 4≤N≤22 πelectrons. We find that the OM2/MRCI descriptions closely resemble those of PariserParrPople (PPP) πelectron models (Tavan and Schulten, Phys. Rev. B 36, 4337, 1987), if equivalent MRCI procedures and regularly alternating model geometries are used. OM2/MRCI optimized geometries are shown to entail improved descriptions particularly for smaller polyenes (Nle 12), for which sizeable deviations from the regular model geometries are found. With CI active spaces covering also the σ in addition to the πelectrons, OM2/MRCI excitation energies turn out to become smaller by at most 0.35 eV for the ionic and 0.15 eV for the covalent excitations. The particlehole (ph) symmetry, which in PPP models arises from the zerodifferential overlap approximation, is demonstrated to be only weakly broken in OM2 such that the oscillator strengths of the covalent 1Bu^{} states, which artificially vanish in phsymmetric models, are predicted to be very small. According to OM2/MRCI and experimental data the 1Bu^{} state is the third excited singlet state for N<12 and becomes the second for N≥14. By comparisons with results of other theoretical approaches and experimental evidence we argue that deficiencies of the particular MRCI method employed by us, which show up in a poor sizeconsistency of the covalent excitations for N>12, are caused by its restriction to at most doubly excited references.
BMO authors (in alphabetic order): Maximilian Schmidt Paul Tavan
Assoziierte Projekte: OM2/MRCI: An efficient QM method to describe electronically excited states of dye molecules
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