Exploring the Role of Decoherence in Condensed-Phase Nonadiabatic Dynamics:  A Comparison of Different Mixed Quantum/Classical Simulation Algorithms for the Excited Hydrated Electron

Abstract

Mixed quantum/classical (MQC) molecular dynamics simulation has become the method of choice for simulating the dynamics of quantum mechanical objects that interact with condensed-phase systems. There are many MQC algorithms available, however, and in cases where nonadiabatic coupling is important, different algorithms may lead to different results. Thus, it has been difficult to reach definitive conclusions about relaxation dynamics using nonadiabatic MQC methods because one is never certain whether any given algorithm includes enough of the necessary physics. In this paper, we explore the physics underlying different nonadiabatic MQC algorithms by comparing and contrasting the excited-state relaxation dynamics of the prototypical condensed-phase MQC system, the hydrated electron, calculated using different algorithms, including: fewest-switches surface hopping, stationary-phase surface hopping, and mean-field dynamics with surface hopping. We also describe in detail how a new nonadiabatic algorithm, mean-field dynamics with stochastic decoherence (MF-SD), is to be implemented for condensed-phase problems, and we apply MF-SD to the excited-state relaxation of the hydrated electron. Our discussion emphasizes the different ways quantum decoherence is treated in each algorithm and the resulting implications for hydrated-electron relaxation dynamics. We find that for three MQC methods that use Tully's fewest-switches criterion to determine surface hopping probabilities, the excited-state lifetime of the electron is the same. Moreover, the nonequilibrium solvent response function of the excited hydrated electron is the same with all of the nonadiabatic MQC algorithms discussed here, so that all of the algorithms would produce similar agreement with experiment. Despite the identical solvent response predicted by each MQC algorithm, we find that MF-SD allows much more mixing of multiple basis states into the quantum wave function than do other methods. This leads to an excited-state lifetime that is longer with MF-SD than with any method that incorporates nonadiabatic effects with the fewest-switches surface hopping criterion.

Does the Hydrated Electron Occupy a Cavity?

“…Figure 4B shows the probability of the hydrated electron remaining excited as a function of time after excitation for our 20-trajectory ensemble; the average excited-state lifetime is~280 fs with a root-mean-squared deviation of~150 fs. Taken together, our calculations predict that photoexcited hydrated electrons return to the ground state much faster than suggested by calculations in which the e − aq resides in a cavity (10,20). Because the underlying relaxation mechanism in our simulations is different, it is important to contrast our assignment of the delayed near-IR transient absorption to those from simulations in which the electron resides in a cavity.…”

Nuclear quantum effects on the nonadiabatic decay mechanism of an excited hydrated electron

“…23 Specifically, the non-equilibrium results obtained here using a fully classical solvent bath correspond well to those obtained from the statistics of full mixed quantum-classical non-adiabatic dynamics simulations published earlier. 19,21 The apparent lifetime in this classical case is a bit less than 1 ps. The results obtained here with a quantum bath provide a decay profile of similar shape, but with a shorter apparent lifetime of ~ 330fs.…”

Cosmic-ray-driven electron-induced reactions of halogenated molecules adsorbed on ice surfaces: Implications for atmospheric ozone depletion and global climate change

“…In 2000s, Researches on electron solvation dynamics and associated dissociative electron transfer reactions of halogenated molecules have continued in water solution [21,[27][28][29][58][59][60][61][62], ice surface [63], ultrathin ice films [16,17,[64][65][66][67] and H 2 O anionic clusters [68][69][70][71][72][73]. Prior to 2008, many experimental and theoretical studies gave very diverse lifetimes and physical natures of e pre states in liquid water [49][50][51][52][53][54][58][59][60][61][62].…”

“…Prior to 2008, many experimental and theoretical studies gave very diverse lifetimes and physical natures of e pre states in liquid water [49][50][51][52][53][54][58][59][60][61][62]. But Wang et al [28] have recently resolved that e pre states are electronically excited states and have lifetimes of ~200 and 500 fs after the identification and removal of a coherent spike effect.…”