On the controversial nature of the 11Bu and 21Bu states of trans-stilbene: the n-electron valence state perturbation theory approach

The nature of two lowest-energy states of B-u symmetry of trans-stilbene and the accurate calculation of their vertical excitation energy have been the subject of a controversy because time dependent density functional theory (TD-DFT) calculations, in agreement with experimental observations, have questioned the results obtained with multireference perturbation theory (MRPT) in the CASPT2 implementation. This paper aims to solve this controversy. By using a different version of MRPT, the n-electron valence state perturbation theory method, the description provided by TD-DFT is confirmed: the lowest B-u singlet state has a HOMO -> LUMO nature and a large oscillator strength (HOMO refers to highest occupied molecular orbital and LUMO refers to lowest unoccupied molecular orbital), while the second B-u singlet state has a mixed HOMO-1 -> LUMO and HOMO -> LUMO+1 nature, has an oscillator strength almost vanishing, and is located at 0.6-0.7 eV higher than the first excited state. The computed vertical excitation energy to the first state (3.8-4.0 eV) is in good agreement with the experimental value.