Evaluation of quantum transition rates from quantum-classical molecular dynamics simulations

Abstract

The impact of quantum decoherence and zero point motion on non-adiabatic transition rates in condensed matter systems is studied in relation to non-adiabatic (NA) molecular dynamics (MD) techniques. Both effects, and decoherence in particular, strongly influence the transition rate, while neither is accounted for by straightforward quantum-classical approaches. Quantum corrections to the quantum-classical results are rigorously introduced based on Kubo’s generating function formulation of Fermi’s Golden rule and the frozen Gaussian approximation for the nuclear wave functions. The development provides a one-to-one correspondence between the decoherence function and the Franck–Condon factor. The decoherence function defined in this paper corrects an error in our previous work [J. Chem. Phys. 104, 5942 (1996)]. The relationship between the short time approach and the real time NA MD is investigated and a specific prescription for incorporating quantum decoherence into NA simulations is given. The proposed scheme is applied to the hydrated electron. The rate of excited state non-radiative relaxation is found to be very sensitive to the decoherence time. Quantum coherence decays about 50% faster in H2O than in D2O, providing a theoretical rationalization for the lack of experimentally observed solvent isotope effect on the relaxation rate. Microscopic analysis of solvent mode contributions to the coherence decay shows that librational degrees of freedom are primarily responsible, due to the strong coupling between the electron and molecular rotations and to the small widths of the wave packets describing these modes. Zero point motion of the O–H bonds decreases the life time of the excited state of the hydrated electron by a factor of 1.3–1.5. The implications of the use of short time approximations for the NA transition rate and for the evolution of the nuclear wave functions are considered.