The elusive excited states of bithiophene: A CASPT2 detective story

A systematic multi-reference perturbation theory investigation of the excitation energies and oscillator strengths for the lowest excited states of 2,2′-bithiophene unequivocally shows that its optical spectrum is produced by two ¹B_u states separated from each other by approximately 1 eV. This picture is confirmed by additional calculations with alternative quantum chemical methods. Our findings are in strong contrast with the previous CASPT2 results of Rubio et al. [J Chem Phys 102:3580 (1995) and Chem Phys Chem 4:1308 (2003)], who predicted that the two lowest ¹B_u states are quasi-degenerate. The methodological reasons responsible for the previous seemingly erroneous assignment of the optical spectrum of bithiophene are identified and explained in terms of unusually large coupling between the ¹B_u states introduced by dynamical correlation effects. A general discussion of applicable computational techniques is offered aiming at avoiding similar problems for other molecular systems.