Evaluating the anharmonicity contributions to the molecular excited state internal conversion rates with finite temperature TD-DMRG.

J Chem Phys

MOE Key Laboratory of Organic OptoElectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People's Republic of China.

Published: June 2021

AI Article Synopsis

  • The authors introduce a new method for calculating molecular nonradiative electronic relaxation rates using time-dependent density matrix renormalization group theory, going beyond traditional harmonic approximation (HA) methods.
  • They find that HA becomes inadequate, particularly at higher excitation energies, where it can either underestimate or overestimate internal conversion rates depending on the potential energy surface's behavior.
  • In practical applications, such as calculating internal conversion rates in azulene, the new method shows significant improvements, yielding rates that are 30%-40% higher than those calculated using HA.

Article Abstract

In this work, we propose a new method to calculate molecular nonradiative electronic relaxation rates based on the numerically exact time-dependent density matrix renormalization group theory. This method could go beyond the existing frameworks under the harmonic approximation (HA) of the potential energy surface (PES) so that the anharmonic effect could be considered, which is of vital importance when the electronic energy gap is much larger than the vibrational frequency. We calculate the internal conversion (IC) rates in a two-mode model with Morse potential to investigate the validity of HA. We find that HA is unsatisfactory unless only the lowest several vibrational states of the lower electronic state are involved in the transition process when the adiabatic excitation energy is relatively low. As the excitation energy increases, HA first underestimates and then overestimates the IC rates when the excited state PES shifts toward the dissociative side of the ground state PES. On the contrary, HA slightly overestimates the IC rates when the excited state PES shifts toward the repulsive side. In both cases, a higher temperature enlarges the error of HA. As a real example to demonstrate the effectiveness and scalability of the method, we calculate the IC rates of azulene from S to S on the ab initio anharmonic PES approximated by the one-mode representation. The calculated IC rates of azulene under HA are consistent with the analytically exact results. The rates on the anharmonic PES are 30%-40% higher than the rates under HA.

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Source
http://dx.doi.org/10.1063/5.0052804DOI Listing

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