05.11.2019 - Jeremy Richardson: Quantum tunnelling, geometric-phase effects and ultrafast nonadiabatic transitions (all using classical molecular dynamics)
| When |
Nov 05, 2019
from 01:00 PM to 02:00 PM |
|---|---|
| Where | HS II, Physical High Rise |
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Quantum tunnelling, geometric-phase effects and ultrafast nonadiabatic transitions (all using classical molecular dynamics)
Standard classical molecular dynamics simulations of molecular systems neglect nuclear quantum effects such as zero-point energy, tunnelling and nonadiabatic transitions. Including these quantum effects exactly would be impossible due to the exponential scaling of quantum mechanics.
In this talk, I will discuss approaches we have developed for describing these quantum effects to a good approximation using extensions of molecular dynamics, which can be efficiently applied to complex molecular systems.
In particular, I will describe semiclassical instanton theory [1], the simplest approach for rigorously describing quantum tunnelling using classical trajectories in imaginary time. We have recently made this method much more efficient using machine-learning tools to interpolate an ab initio potential-energy surface along the tunnelling pathway [2]. An extension to include the geometric-phase effect allows us to explain the high-resolution experimental spectrum of the methane cation and its isotopologues.
In order to simulate nonadiabatic transitions, we employ the Meyer-Miller-Stock-Thoss mapping approach [3]. Unfortunately the simplest quasiclassical methods based on this idea show significant errors when applied to predict population transfer in asymmetric systems. We show that these problems can be fixed, without modifying the dynamics, simply by changing the operators used to describe observables [4,5]. We demonstrate a significant improvement on spin-boson models, the 7-state FMO exciton model and a vibronic-coupling model of singlet-fission.
[1] J. O. Richardson, J. Chem. Phys. 148, 200901 (2018)
[2] G. Laude, D. Calderini, D. P. Tew & J. O. Richardson, Faraday
Discuss. 212, 237 (2018)
[3] G. Stock & M. Thoss, Phys. Rev. Lett. 78, 578 (1997)
[4] M. A. C. Saller, A. Kelly & J. O. Richardson, J. Chem. Phys. 150,
071101 (2019)
[5] J. E. Runeson & J. O. Richardson, J. Chem. Phys. 151, 044119 (2019)
