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WIREs Comput Mol Sci
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First‐principles dynamics of photoexcited molecules and materials towards a quantum description

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Abstract The past decades have witnessed the success of ground‐state density functional theory capturing static electronic properties of various materials. However, for time dependent processes especially those involving excited states, real‐time time‐dependent density functional theory (rt‐TDDFT) and advanced nonadiabatic algorithms are essential, especially for practical simulations of molecules and materials under the occurrence of ultrafast laser field. Here we summarize the recent progresses in developing rt‐TDDFT approaches within numerical atomic orbitals and planewave formalisms, as well as the efforts combining rt‐TDDFT and ring polymer molecular dynamics to take into account nuclear quantum effects in quantum electronic‐nuclear dynamic simulations. Typical applications of first‐principles dynamics of excited electronic states including high harmonic generation, charge density wave, photocatalytic water splitting, as well as quantum nuclear motions in ozone and graphene, are presented to demonstrate the features and advantages of these methods. The progresses in method developments and practical applications provide unprecedented insights into nonadiabatic dynamics of excited states in the Ehrenfest scheme and beyond, towards a comprehensive understanding of excited electronic structure, electron–phonon interactions, photoinduced charge transfer and chemical reactions, as well as quantum nuclear motions in excited states. This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods Electronic Structure Theory > Density Functional Theory Molecular and Statistical Mechanics > Molecular Dynamics and Monte‐Carlo Methods
(a) Schematic diagram of the photoinduced electron–hole excitation and relaxation processes. The electron hole occupation numbers of graphene as a function of time with the classical and quantum treatments of nuclear motion are shown in (b) and (c), respectively. Dashed lines are the exponential fitting to the hole occupation number (Reprinted with permission from Reference 36. Copyright 2019 IOP Publishing Ltd)
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(a) Atomic configurations of ozone in the normal and cyclic states. (b) The number of excited electrons in each bead of ozone molecule as a function of time at 20 K. (c) Temporal distance of three oxygen atoms in each bead at 20 K. We separate all beads into normal (blue) and cyclic states (red), according to a geometrical criterion (Reprinted with permission from Reference 36. Copyright 2019 IOP Publishing Ltd)
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(a) Time evolved OH bond length in single water splitting and four typical snapshots (27, 65, 70, and 86 fs) in the first‐principles dynamics. (b) The evolutions of electrons numbers on each species by Hirshfeld charge analysis in 0–27 fs. (c) 55–59 fs. (d) 59–70 fs. We treat the initial state as a zero‐electron state and scale the results 10 times, defining this 10‐fold charge as pseudo electron
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(a) Snapshot of the Au20 cluster in water, where yellow, red, and gray spheres represent gold, oxygen, and hydrogen atoms, respectively. The arrow denotes polarization direction of the laser field. (b) Time evolution of the laser field with field strength Emax = 2.3 V/Å and frequency ħω = 2.81 eV. Under this laser pulse, time‐evolved OH bond length dOH of all water molecules with (c) and without (d) Au20 cluster are shown. (e) Atomic configurations at time t = 0, 16, 18, and 21 fs (Reprinted with permission from Reference 40. Copyright 2018 American Chemical Society)
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Schematic of atomic processes in photoexcited 1T‐TiSe2. The laser pulse melts charge order within 20 fs, producing the forces that trigger the ionic movements. The self‐amplified dynamics is assisted by electron–phonon couplings after initial excitation (Reprinted with permission from Reference 39. Copyright 2019 Springer Nature)
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. Time evolution of atomic structures of bulk 1T‐TaS2 under different photoexcitation. (a) Snapshots of time‐dependent atomic structures for η = 0.64%, η = 1.28%, and η = 1.92% at 0, 250, 480, and 840 fs after photoexcitation, respectively. (b) Evolution of RMSD under three laser intensities (black line, η = 0.64%; purple line, η = 1.28%; red line, η = 1.92%). (c) Corresponding evolution of the ionic temperatures calculated from the kinetic energy of all ions at different times (Reprinted with permission from Reference 38. Copyright 2019 American Chemical Society)
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. Strain‐dependent HHG yield for different harmonics of 1L‐MoS2. (a) Evolution of the normalized HHG spectrum under tensile and compressive strain. (b) The relative change in HHG intensity as a function of strain for representative harmonics (colored dots) and the linear fit (solid lines). (c) Applying the same method to all harmonics, the absolute value of the slope changes nearly periodically. Black and red arrows labeled as X and X′ (X = A, B, …) denote the harmonics in the first and second cycle (Reprinted with permission from Reference 37. Copyright 2019 American Physical Society)
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. (a) Flowchart of k‐resolved algorithm for evolution of electronic system. Here Sk, Hk, and unk are the overlap matrix, Hamiltonian matrix, and TDKS orbitals at k, respectively. (Reprinted with permission from Reference 33. Copyright 2018 Wiley & Sons, Inc.) (b) The scheme of quantum dynamics for electronic and nuclear evolution. At first, electrons are excited by light field and ions move on an excited potential energy surface (S1). Considering the nuclear quantum effect, the wavefunction of nuclei may separate into a few wave packets and exhibit quantum behaviors upon the impact of electronic excitation. (Reprinted with permission from Reference 36. Copyright 2019 IOP Publishing Ltd) (c) Flowchart of RP‐TDAP in practical computation. RPMD evolves the atomic positions and rt‐TDDFT manages the evaluation of atomic forces. Each bead follows its own electron‐nuclear evolution, while all beads are connected by a harmonic spring to neighboring beads (Reprinted with permission from Reference 36. Copyright 2019 IOP Publishing Ltd)
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Molecular and Statistical Mechanics > Molecular Dynamics and Monte-Carlo Methods
Electronic Structure Theory > Density Functional Theory
Electronic Structure Theory > Ab Initio Electronic Structure Methods

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