The Ultrafast Photochemistry of Thymine: Predictions From Coupled Cluster Theory and the Hole-Hole Tamm-Dancoff Approximation
Otto Fajen, Stanford University
The ultrafast photorelaxation of thymine upon UV excitation plays a significant role in the resilience of our genetic material to light-induced DNA damage and has been studied extensively, both experimentally and theoretically. A recent TRXAS experiment confirmed the ultrafast population of the optically dark npi* state and was underpinned by Coupled Cluster (CC) theory calculations, which achieve a high-level of accuracy for many systems. In the present work, we implemented the first CC-based nonadiabatic dynamics framework with a proper treatment of conical intersections- a known deficiency of standard CC theories- and applied this framework to the ultrafast relaxation of thymine. We simulated the TRXAS based upon these dynamics and obtained quantitative agreement with the previous experiment. Further, we observed an unexpected N-H dissociation involving a pisigma* state, similar to a known channel in adenine. Because CC methods are computationally demanding and formally unable to describe intersections between ground and excited states, we supplemented the initial 100 femtoseconds of CC dynamics with a 2 picosecond study using the hole-hole Tamm-Dancoff approximation (hh-TDA) method, which offers a convenient combination of low computational expense, good performance relative to EOM-CCSD, and the ability to treat nonradiative decay back to the ground state. We computed TRXAS and compare to the CC results as well as experiment. This combination of high- and low-level dynamics allows us to treat the full photorelaxation process, from S2 all the way down to S0 with high accuracy and high confidence.