Electronic states of the benzene dimer: A simple case of complexity
Electronic structure calculations of the excited states of the benzene dimer using equation-of-motion coupled-cluster method are reported. The calculations reveal large density of electronic states including multiple valence, Rydberg and mixed Rydberg-valence states. The calculations of the oscillator strengths for the transitions between the excimer state and other excited states allowed us to identify the target state responsible for the excimer absorption as the E1u state of a mixed Rydberg-valence character at 3.04 eV above the excimer (11B1g). Although at D6h, the 11B1g to E1u transition is symmetry-forbidden, small geometric displacements (to D2h) that have negligible effect on excitation energy split this degenerate state into the dark (4B3u) and bright (4B2u) components (oscillator strength of 0.3).