More Like this

1. Total angular momentum conservation in Ehrenfest dynamics with a truncated basis of adiabatic states

   https://doi.org/10.1063/5.0177778  

   Tao, Zhen; Bian, Xuezhi; Wu, Yanze; Rawlinson, Jonathan; Littlejohn, Robert G; Subotnik, Joseph E (February 2024, The Journal of Chemical Physics)

   We show that standard Ehrenfest dynamics does not conserve linear and angular momentum when using a basis of truncated adiabatic states. However, we also show that previously proposed effective Ehrenfest equations of motion [M. Amano and K. Takatsuka, “Quantum fluctuation of electronic wave-packet dynamics coupled with classical nuclear motions,” J. Chem. Phys. 122, 084113 (2005) and V. Krishna, “Path integral formulation for quantum nonadiabatic dynamics and the mixed quantum classical limit,” J. Chem. Phys. 126, 134107 (2007)] involving the non-Abelian Berry force do maintain momentum conservation. As a numerical example, we investigate the Kramers doublet of the methoxy radical using generalized Hartree–Fock with spin–orbit coupling and confirm that angular momentum is conserved with the proper equations of motion. Our work makes clear some of the limitations of the Born–Oppenheimer approximation when using ab initio electronic structure theory to treat systems with unpaired electronic spin degrees of freedom, and we demonstrate that Ehrenfest dynamics can offer much improved, qualitatively correct results. 

   more » « less
2. Nonadiabatic dynamics with classical trajectories: The problem of an initial coherent superposition of electronic states

   https://doi.org/10.1063/5.0186984  

   Villaseco_Arribas, Evaristo; Maitra, Neepa T; Agostini, Federica (February 2024, The Journal of Chemical Physics)

   Advances in coherent light sources and development of pump–probe techniques in recent decades have opened the way to study electronic motion in its natural time scale. When an ultrashort laser pulse interacts with a molecular target, a coherent superposition of electronic states is created and the triggered electron dynamics is coupled to the nuclear motion. A natural and computationally efficient choice to simulate this correlated dynamics is a trajectory-based method where the quantum-mechanical electronic evolution is coupled to a classical-like nuclear dynamics. These methods must approximate the initial correlated electron–nuclear state by associating an initial electronic wavefunction to each classical trajectory in the ensemble. Different possibilities exist that reproduce the initial populations of the exact molecular wavefunction when represented in a basis. We show that different choices yield different dynamics and explore the effect of this choice in Ehrenfest, surface hopping, and exact-factorization-based coupled-trajectory schemes in a one-dimensional two-electronic-state model system that can be solved numerically exactly. This work aims to clarify the problems that standard trajectory-based techniques might have when a coherent superposition of electronic states is created to initialize the dynamics, to discuss what properties and observables are affected by different choices of electronic initial conditions and to point out the importance of quantum-momentum-induced electronic transitions in coupled-trajectory schemes. 

   more » « less
3. Exciton-Assisted Relaxation of Hot Carriers in Semiconductor Quantum Dots

   https://doi.org/10.1149/MA2024-01231396mtgabs  

   Griffin, Hadassah B; Kryjevski, Andrei B; Kilina, Svetlana V; Kilin, Dmitri S (August 2024, ECS Meeting Abstracts)

   The work provides computational arguments in support of excitonic approach for the treatment of the photo-induced processes in semiconductor quantum dots. The non-radiative relaxation, non-radiative recombination, and photo-luminescence quantum yield are computed for a range of atomistic models of semiconductor quantum dots (QDs) in the quantum confinement regime. The excitonic (EX) approach is compared to independent orbital approximation (IOA) approach. Both approaches address dissipation of the electronic energy from electronic degrees of freedom to thermal vibrations of the lattice. The difference of two approaches appears in treatment of energies of electronic states and in a way how the electron-phonon interaction is taken into account. IOA approach uses energies of Kohn-Sham orbitals and on the fly non-adiabatic couplings. [1-3] EX approach uses Bethe-Salpeter equation (BSE) for energies.[4-6] The excitonic wavefunctions from BSE is used to construct a linear transformation matrix that transforms IOA-based non-adiabatic couplings into an excitonic basis. Both approaches are compared in application to untrasmall 1 nm diameter Si QD.Results include an evidence that hot excitons relax sooner in the excitonic picture than in the IOA picture. The observed effect is rationalized via smaller subgaps and different available relaxation pathways in the excitonic picture. The most surprising result is found for the simulated emission spectrum. The spectum in the excitonic picture demonstrates intensity in several 5 orders of magnitude higher than in the IOA picture. This observation is related to formation of a bright exciton in the lowest excitation of the ultra-small Si QDs. Obtained evidence favors excitonic approach and promises a reliable interpretation and prediction of time-dependent observables in a range of semiconductor quantum dots of different composition, sizes, and surface environment.[7] Most intriguing results are expected for QDs representing interface between PbSe and CdSe. [8] Support of National Science foundation via NSF CHE-2004197 is gratefully acknowledged. [1] D. S. Kilin and D. A. Micha, “Relaxation of photoexcited electrons at a nanostructured Si(111) surface”, J. Phys. Chem. Lett. 1, 1073-1077 (2010). [2] D. J. Vogel and D. S. Kilin, "First-Principles Treatment of Photoluminescence in Semiconductors" J. Phys. Chem. C 119, 50, 27954–27964 (2015). [3] D. J. Vogel, A. B. Kryjevski, T. M. Inerbaev, and D. S. Kilin, "Photoinduced Single- and Multiple-Electron Dynamics Processes Enhanced by Quantum Confinement in Lead Halide Perovskite Quantum Dots", J. Phys. Chem. Lett. 8, 13, 3032–3039(2017). [4] A. B. Kryjevski and Dmitri Kilin, "Enhanced multiple exciton generation in amorphous silicon nanowires and films", Molec. Phys. 114, 365-379 (2016). [5] M. Rohlfing and S. G. Louie, "Electron-Hole Excitations in Semiconductors and Insulators", Phys. Rev. Lett. 81, 2312-2315 (1998). [6] T. Sander, G. Kresse, "Macroscopic dielectric function within time-dependent density functional theory—Real time evolution versus the Casida approach", J. Chem. Phys. 146, 064110 (2017). [7] S. V. Kilina, P. K. Tamukong, and D. S. Kilin, "Surface Chemistry of Semiconducting Quantum Dots: Theoretical Perspectives", Acc. Chem. Res. 49, 10, 2127–2135 (2016). [8] H. B. Griffin, A. B. Kryjevski, and Dmitri S. Kilin, "Ab initio calculations of through-space and through-bond charge-transfer properties of interacting Janus-like PbSe and CdSe quantum dot heterostructures", Molec. Phys., e2273415 (2023). 

   more » « less
4. Nonadiabatic molecular dynamics with subsystem density functional theory: application to crystalline pentacene

   https://doi.org/10.1088/1361-648X/ad577d  

   Zhang, Qingxin; Shao, Xuecheng; Li, Wei; Mi, Wenhui; Pavanello, Michele; Akimov, Alexey_V (June 2024, Journal of Physics: Condensed Matter)

   Abstract In this work, we report the development and assessment of the nonadiabatic molecular dynamics approach with the electronic structure calculations based on the linearly scaling subsystem density functional method. The approach is implemented in an open-source embedded Quantum Espresso/Libra software specially designed for nonadiabatic dynamics simulations in extended systems. As proof of the applicability of this method to large condensed-matter systems, we examine the dynamics of nonradiative relaxation of excess excitation energy in pentacene crystals with the simulation supercells containing more than 600 atoms. We find that increased structural disorder observed in larger supercell models induces larger nonadiabatic couplings of electronic states and accelerates the relaxation dynamics of excited states. We conduct a comparative analysis of several quantum-classical trajectory surface hopping schemes, including two new methods proposed in this work (revised decoherence-induced surface hopping and instantaneous decoherence at frustrated hops). Most of the tested schemes suggest fast energy relaxation occurring with the timescales in the 0.7–2.0 ps range, but they significantly overestimate the ground state recovery rates. Only the modified simplified decay of mixing approach yields a notably slower relaxation timescales of 8–14 ps, with a significantly inhibited ground state recovery. 

   more » « less
5. Excited State Dynamics in Silicon Carbide Nanostructures

   Graupner, David; Granlie, Joseph; Roeschlein, Eric; Han, Yulun; Hobbie, Erik; Kilin, Dmitri (February 2025, 64th Sanibel Symposium, Quantum Theory Project, University of Florida)

   A recent interest to photoexcited processes in silicon carbide (SiC) nanomaterials calls for a range of excited state methods to interpret and predict optoelectronic properties of this class of materials as a function of their structural features. The optoelectronic response to electromagnetic field in visible and infrared ranges requires case-specific approximations. Excited state dynamics modeling of ensembles of precursors molecules (methylsilane, cyclohexasilane,1 etc. demonstrates a sequence of bond breaking and formation leading to growth of SiC nanostructures.2 Open quantum system treatment of 2D and 3D SiC nanostructures allows to interpret competition of energy dissipation into lattice vibrations versus radiative recombination. Such treatment identifies ability of a specific nanostructure to serve as an efficient light emitter in the visible range. Redfield formalism for dissipative quantum relaxation parameterized by on-the-fly nonadiabatic couplings3 allows to interpret a surprising absence of luminescence quenching by oxygen contamination, in agreement with experiment.2,4 Same method predicts high photoluminescence quantum yield of 2D SiC nanosheets with VcVsi divacancies. Quantization of nuclear motion of surface ligands of SiC nanostructures near surface defects offers interpretation of PL spectra substructure, confirmed by experiment. A machine learning approach allows to summarize trends in quantum relaxation for a range of SiC models with a distribution of morphological parameters. Exploration of response of SiC nanostructures to the mid-infrared range excitation is addressed by modeling coupled dynamics of electronic, nuclear, and resonator mode degrees of freedom. An effect of dynamical polarization and collective motion of charge density is modeled in the frequency range of SiC optical phonons. A dependence on concentration of charge carriers and surface termination is discussed. Modeling of optoelectronic properties of SiC nanostructures allows to evaluate their potential for telecommunication applications. (1) Han, Y.; et. al., J. Phys. Chem. Lett. 2018, 9 (15), 4349-4354. (2) Granlie, J. D.; et. al., J. Phys. Chem. Lett. 2023, 14 (26), 6202-6208. (3) Kilin, D. S.; Micha, D. A., J. Phys. Chem. Lett. 2010, 1,(7) 1073–1077. (4) Thomas, S. A.; et. al., ACS Nano 2024, 18 (39), 26848-26857. 

   more » « less