Computational simulation of nonadiabatic molecular dynamics is an indispensable tool for understanding complex photoinduced processes such as internal conversion, energy transfer, charge separation, and spatial localization of excitons, to name a few. We report an implementation of the fewest-switches surface-hopping algorithm in the NWChem computational chemistry program. The surface-hopping method is combined with linear-response time-dependent density functional theory calculations of adiabatic excited-state potential energy surfaces. To treat quantum transitions between arbitrary electronic Born-Oppenheimer states, we have implemented both numerical and analytical differentiation schemes for derivative nonadiabatic couplings. A numerical approach for the time-derivative nonadiabatic couplings together with an analytical method for calculating nonadiabatic coupling vectors is an efficient combination for surface-hopping approaches. Additionally, electronic decoherence schemes and a state reassigned unavoided crossings algorithm are implemented to improve the accuracy of the simulated dynamics and to handle trivial unavoided crossings. We apply our code to study the ultrafast decay of photoexcited benzene, including a detailed analysis of the potential energy surface, population decay timescales, and vibrational coordinates coupled to the excitation dynamics. We also study the photoinduced dynamics in trans-distyrylbenzene. This study provides a baseline for future implementations of higher-level frameworks for simulating nonadiabatic molecular dynamics in NWChem.
Bibliographical noteFunding Information:
H.S., S.T., N.G., and S.M. acknowledge support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under Contracts No. KC0301030, KC030103172684, and award no. DE-SC0019484. S.T. acknowledges the support of the Center for Integrated Nanotechnology (CINT) at Los Alamos National Laboratory (LANL), a U.S. Department of Energy and Office of Basic Energy Sciences User Facility. S.A.F. acknowledges support from the U.S. Office of Naval Research through the U.S. Naval Research Laboratory. S.A.F, N.G., and C.J.C. also acknowledge support by the U.S. Department of Energy, Office of Science, Office of Advanced Scientific Computing Research, Scientific Discovery through Advanced Computing (SciDAC) program (2013–2017) under Award Nos. KC030102062653 (S.A.F. and N.G.) and DESC0008666 (C.J.C.) (Charge Transfer and Charge Transport in Photoactivated Systems), a University of Minnesota/LBNL/PNNL partnership, under which this NAMD implementation in NWChem was initially begun. This research used resources provided by the LANL Institutional Computing Program and also benefited from computational resources provided by EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle Memorial Institute for the United States Department of Energy under DOE contract number DE-AC05-76RL1830.
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