Theoretical study of the singlet and triplet vertical electronic transitions of styrene by the symmetry adapted cluster-configuration interaction method

The ground state, singlet! singlet and singlet! triplet vertical excited states of styrene have been studied by using the symmetry adapted cluster-configuration interaction (SAC-CI) method with aug-cc-pVDZ basis sets supplemented with molecule-centered Rydberg functions. The characteristic structures of the bands observed in the vacuum ultraviolet (VUV) and electron energy loss (EEL) spectra have been theoretically clarified by calculating the excitation energies, oscillator strengths, and second moments for all the excited states in the energy region 2.6–7.0 eV. The present SAC-CI theoretical results, including both the singlet! singlet and singlet! triplet vertical electronic transitions, have well reproduced the profile of the electronic spectra of styrene. Higher-energy singlet! triplet excited states and Rydberg states were extensively addressed. The valence and Rydberg configurations were found strongly mix with each other in the 61A0 and 81A0 excited states at the SAC-CI/aug-cc-pVDZ(R) level. Three main differences were found comparing with the previous CASPT2 study. (1) The 3s0and 3p-series of Rydberg states were predicted lower than the 51A0 and 61A0 valence p–p excited states in the SAC-CI study, whereas higher than the 51A0 and 61A0 excited states in the CASPT2 study. (2) The valence excited state (41A0) involving doubly excited configurations (4a00; 4a00)! (5a00; 5a00) computed at 6.08 eV at the CASPT2 level was not found in the SAC-CI calculation. (3) The characterizations (main configurations) of the 51A0 and 61A0 excited states have been found to exchange their components in the SAC-CI and CASPT2 calculations. 2004 Elsevier B.V. All rights reserved.