Internal Conversion between Bright (11Bu+) and Dark (21Ag–) States in s-trans-Butadiene and s-trans-Hexatriene

• DOI: 10.1021/acs.jpclett.1c02707
• Publication Date: 2021-09-30T18:05:44Z
• Authors: Woojin Park; Jun Shen; Seunghoon Lee; Piotr Piecuch; Michael Filatov; Cheol Ho Choi
• Keywords: Coupled cluster

Internal conversion (IC) between the two lowest singlet excited states, 11Bu+ and 21Ag-, of s-trans-butadiene s-trans-hexatriene is investigated using a series single- multi- reference wave function density functional theory (DFT) methodologies. Three independent types equation-of-motion coupled-cluster (EOMCC) capable providing an accurate balanced description one- as well two-electron transitions, abbreviated δ-CR-EOMCC(2,3), DIP-EOMCC(4h2p){No}, DEA-EOMCC(4p2h){Nu} or DEA-EOMCC(3p1h,4p2h){Nu}, consistently predict that 11Bu+/21Ag- crossing in both molecules occurs along bond length alternation coordinate. However, analogous 21Ag- potentials obtained with some multireference approaches, such CASSCF MRCIS(D), linear-response formulation time-dependent DFT (TDDFT), do not cross. Hence, caution needs to be exercised when studying low-lying states polyenes conventional multiconfigurational methods TDDFT. The multistate many-body perturbation methods, XMCQDPT2, correctly reproduce curve crossing. Among simplest least expensive computational methodologies, approaches incorporate contributions doubly configurations, MRSF (mixed spin-flip) TDDFT SSR(4,4), accurately our best EOMCC results. This highly promising for nonadiabatic molecular dynamics simulations larger systems.