The 2 1A g–1 1B u energy gap in the polyenes: An extended configuration interaction study

For a correct account of the ordering of excited states in the polyenes, in particular the low-lying 2 1Ag and 1 1Bu states, single as well as double excited configurations must be included in a CI expansion. However, for longer π systems such expansion shows, in contrast to spectroscopic observations, a divergence of excitation energies and a reversal of the 2 1Ag and 1 1Bu state ordering. We have therefore extended the CI expansion to include all triple and quadruple excitations for the polyene series CnHn+2’ n=4,6,8,10,12 and n=4,6,8,10, respectively. In our calculations we employed a PPP model Hamiltonian. The extended CI expansions correct the faults of previous treatments and predict an increase of the 2 1Ag–11Bu energy gap with increasing polyene length in agreement with recent spectroscopic observations. The effect of higher excitations is mainly due to triple excitations involving three simultaneous spin flips of ground state electrons.