Assessing the Performance of CC2 for Excited-State Dynamics: A Benchmark Study with Pyrazine

Abstract

During the ultrafast electronic relaxation of pyrazine, the bright state undergoes rapid decay accompanied by coupled electronic and nuclear motion. We investigate the vibrational modes associated with this process using trajectory surface hopping simulations. To enable accurate \emph{ab initio} excited-state dynamics, we apply CC2 analytical gradients and nonadiabatic coupling vectors within the Q-Chem package. Using both a reduced vibronic coupling model and full-dimensional simulations, we demonstrate that CC2 predicts that the dark $A_{\text{1u}}$ state actively participates in the initial relaxation and the subsequent dynamics. The full-dimensional simulations yield a $B_{\text{2u}}$ decay time of 25--26 fs, in good agreement with the experimental measurement of $22\pm 3$ fs. Furthermore, we identify the $Q_{\text{8a}}$ and $Q_{\text{9a}}$ modes as dominant tuning coordinates whose motions are phase-locked to the population oscillations between the $A_{1u}$ and $B_{3u}$ states.

arXiv commentMain text: 39 pages, 6 figures; Supporting information: 36 pages, 18 figures