A revised MRCI-algorithm coupled to an effective valence-shell Hamiltonian. II. Application to the valence excitations of butadiene

In Paper I of this work we have sketched an improved MRCI algorithm and its coupling to the effective valence-shell Hamiltonian OM2. To check the quality of the resulting OM2/MRCI approach, it is applied here to the excited valence states of all-trans butadiene. As is explained by a review of previous theoretical work, proper descriptions of these states posed severe problems within correlated ab initio treatments but seemed to be trivial within simple correlated pi-electron models. We now show that an extended MRCI treatment of the correlations among all valence electrons as described by OM2 closely reproduces the experimental evidence, placing the vertical 2 (1)A(g) excitation by about 0.2 eV below the 1 B-1(u) excitation. By an analysis of sigma-pi interactions we explain the corresponding earlier success of correlated pi-electron theory. Exploiting the enhanced capabilities of the new approach we investigate the potential surfaces. Here, OM2/MRCI is shown to predict that the 2 (1)A(g) state is energetically lowered about four times more strongly than the 1 B-1(u) state upon geometry relaxation constrained to the C-2h symmetry. We conclude that OM2/MRCI should be well-suited for the study of excited state surfaces of organic dye molecules. (C) 2002 American Institute of Physics.