Nonadiabatic Kinetics in the Intermediate Coupling Regime: Comparing Molecular Dynamics to an Energy-Grained Master Equation

We
propose and test an extension of the energy-grained master equation
(EGME) for treating nonadiabatic (NA) hopping between different potential
energy surfaces, which enables us to model the competition between
stepwise collisional relaxation and kinetic processes which transfer
population between different electronic states of the same spin symmetry.
By incorporating Zhu–Nakamura theory into the EGME, we are
able to treat NA passages beyond the simple Landau–Zener approximation,
along with the corresponding treatments of zero-point energy and tunneling
probability. To evaluate the performance of this NA-EGME approach,
we carried out detailed studies of the UV photodynamics of the volatile
organic compound C6-hydroperoxy aldehyde (C6-HPALD) using on-the-fly ab initio molecular dynamics and trajectory
surface hopping. For this multichromophore molecule, we show that
the EGME is able to capture important aspects of the dynamics, including
kinetic timescales, and diabatic trapping. Such an approach provides
a promising and efficient strategy for treating the long-time dynamics
of photoexcited molecules in regimes which are difficult to capture
using atomistic on-the-fly molecular dynamics.