Abstract
Within the fewest switches surface hopping (FSSH) formulation, a swarm of independent trajectories is propagated and the equations of motion for the quantum coefficients are evolved coherently along each independent nuclear trajectory. That is, the phase factors, or quantum amplitudes, are retained. At a region of strong coupling, a trajectory can branch into multiple wavepackets. Directly following a hop, the two wavepackets remain in a region of nonadiabatic coupling and continue exchanging population. After these wavepackets have sufficiently separated in phase space, they should begin to evolve independently from one another, the process known as decoherence. Decoherence is not accounted for in the standard surface hopping algorithm and leads to internal inconsistency. FSSH is designed to ensure that at any time, the fraction of classical trajectories evolving on each quantum state is equal to the average quantum probability for that state. However, in many systems this internal consistency requirement is violated. Treating decoherence is an inherent problem that can be addressed by implementing some form of decoherence correction to the standard FSSH algorithm. In this study, we have implemented two forms of the instantaneous decoherence procedure where coefficients are reinitialized following hops. We also test the energy-based decoherence correction (EDC) scheme proposed by Granucci et al. and a related version where the form of the decoherence time is taken from Truhlar's Coherent Switching with Decay of Mixing method. The sensitivity of the EDC results to changes in parameters is also evaluated. The application of these computationally inexpensive ad hoc methods is demonstrated in the simulation of nonradiative relaxation in two conjugated oligomer systems, specifically poly-phenylene vinylene and poly-phenylene ethynylene. We find that methods that have been used successfully for treating small systems do not necessarily translate to large polyatomic systems and their success depends on the particular system under study.
Highlights
Molecular dynamics with quantum transitions (MDQT) is a well tested, computationally efficient, conceptually simple, and accurate method for the simulation of nonadiabatic processes in which the nuclear and electronic systems are treated separately
In order to investigate decoherence effects, we have performed nonadiabatic excited-state molecular dynamics (NA-ESMD) simulations of the photoinduced dynamics of the two model systems depicted in Fig. 2: A 3-ring oligomer of poly-phenylene vinylene (PPV) (PPV3), and a system composed of metasubstituted linear poly-phenylene ethynylene (PPE) segments of 2, 3, and 4-rings (2-3-4 PPE)
NA-ESMD simulations were performed on PPV3 and 23-4 PPE to model the nonradiative relaxation following photoexcitation to a high energy excited state at room temperature
Summary
Molecular dynamics with quantum transitions (MDQT) is a well tested, computationally efficient, conceptually simple, and accurate method for the simulation of nonadiabatic processes in which the nuclear and electronic systems are treated separately. In the fewest switches surface hopping (FSSH) scheme, described by Tully, the nuclei are treated classically while the electrons are treated within the quantum mechanical framework, and transitions between multiple coupled excited states are allowed depending on the strength of the nonadiabatic coupling. The nuclei are evolved according to forces governed by the potential energy surface (PES) of a single adiabatic electronic excited state, the “current state.”. We have developed a nonadiabatic excited-state molecular dynamics (NA-ESMD) framework capable of extending the FSSH formalism to large polyatomic molecules with many coupled electronic states. We have investigated the dependence of simulation results on convergence and parameters and the success of NA-ESMD in modeling photoinduced dynamics including nonradiative relaxation and energy transfer has been demonstrated for a variety of large polyatomic systems.. We have investigated the dependence of simulation results on convergence and parameters and the success of NA-ESMD in modeling photoinduced dynamics including nonradiative relaxation and energy transfer has been demonstrated for a variety of large polyatomic systems. In this paper, we continue to develop our NAESMD methodology by analyzing the affect of various ad hoc decoherence corrections
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