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

Abstract

For a correct account of the ordering of excited states in the polyenes, in particular the low‐lying 2 ¹A_g and 1 ¹B_u 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 ¹A_g and 1 ¹B_u state ordering. We have therefore extended the CI expansion to include all triple and quadruple excitations for the polyene series C_nH_n+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 ¹A_g–1¹B_u 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.