Nonadiabatic Molecular Dynamics Simulations Research Articles

Surface Hydroxylation during Water Splitting Promotes the Photoactivity of BiVO4(010) Surface by Suppressing Polaron-Mediated Charge Recombination.

Polaron-based electron transport restricts the photoelectrochemical (PEC) water splitting efficiency of BiVO4. However, the location and dynamics of polarons are significantly dependent on the surface hydroxylation. By performing ab initio nonadiabatic molecular dynamics simulations, we demonstrated that hydroxylation of BiVO4(010) surface greatly alleviates the detrimental effect of oxygen-vacancy-induced electron polaron (EP). Surface hydroxylation stabilizes the EP at the surface to facilitate water splitting, makes the polaron a shallow localized state, and reduces the intensity of high-frequency V-O bond stretching vibrations. By decreasing the nonadiabatic coupling and decoherence time, the charge carrier lifetimes are extended by 1-3 orders of magnitude depending on the hydroxylation coverage. Our study not only reveals that the surface hydroxylation mitigated detrimental impacts of polarons in metal oxides but also provided valuable insights into the benign effect of intermediate species on the photocatalytic reactivity.

Real-Time Dynamics and Detailed Balance in Ring Polymer Surface Hopping: The Impact of Frustrated Hops.

Ring polymer surface hopping (RPSH) has been recently introduced as a well-tailored method for incorporating nuclear quantum effects, such as zero-point energy and tunneling, into nonadiabatic molecular dynamics simulations. The practical widespread usage of RPSH demands a comprehensive benchmarking of different reaction regimes and conditions with equal emphasis on demonstrating both the cons and the pros of the method. Here, we investigate the fundamental questions related to the conservation of energy and detailed balance in the context of RPSH. Using Tully's avoided crossing model as well as a 2-state quantum system coupled to a classical bath undergoing Langevin dynamics, we probe the critical problem of the proper treatment of the classically forbidden transitions stemming from the surface hopping algorithm. We show that proper treatment of these frustrated hops is key to the accurate description of real-time dynamics as well as reproducing the correct quantum Boltzmann populations.

Transiently delocalized states enhance hole mobility in organic molecular semiconductors.

Evidence shows that charge carriers in organic semiconductors self-localize because of dynamic disorder. Nevertheless, some organic semiconductors feature reduced mobility at increasing temperature, a hallmark for delocalized band transport. Here we present the temperature-dependent mobility in two record-mobility organic semiconductors: dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]-thiophene (DNTT) and its alkylated derivative, C8-DNTT-C8. By combining terahertz photoconductivity measurements with atomistic non-adiabatic molecular dynamics simulations, we show that while both crystals display a power-law decrease of the mobility (μ) with temperature (T) following μ ∝ T -n, the exponent n differs substantially. Modelling reveals that the differences between the two chemically similar semiconductors can be traced to the delocalization of the different states that are thermally accessible by charge carriers, which in turn depends on their specific electronic band structure. The emerging picture is that of holes surfing on a dynamic manifold of vibrationally dressed extended states with a temperature-dependent mobility that provides a sensitive fingerprint for the underlying density of states.

Ab initio nonadiabatic molecular dynamics study on spin–orbit coupling induced spin dynamics in ferromagnetic metals

Understanding the photoexcitation induced spin dynamics in ferromagnetic metals is important for the design of photo-controlled ultrafast spintronic device. In this work, by the ab initio nonadiabatic molecular dynamics simulation, we have studied the spin dynamics induced by spin–orbit coupling (SOC) in Co and Fe using both spin-diabatic and spin-adiabatic representations. In Co system, it is found that the Fermi surface (E F) is predominantly contributed by the spin-minority states. The SOC induced spin flip will occur for the photo-excited spin-majority electrons as they relax to the E F, and the spin-minority electrons tend to relax to the E F with the same spin through the electron–phonon coupling (EPC). The reduction of spin-majority electrons and the increase of spin-minority electrons lead to demagnetization of Co within 100 fs. By contrast, in Fe system, the E F is dominated by the spin-majority states. In this case, the SOC induced spin flip occurs for the photo-excited spin-minority electrons, which leads to a magnetization enhancement. If we move the E F of Fe to higher energy by 0.6 eV, the E F will be contributed by the spin-minority states and the demagnetization will be observed again. This work provides a new perspective for understanding the SOC induced spin dynamics mechanism in magnetic metal systems.

In-Situ-Grown Cu Dendrites Plasmonically Enhance Electrocatalytic Hydrogen Evolution on Facet-Engineered Cu2 O.

Herein, facet-engineered Cu2 O nanostructures are synthesized by wet chemical methods for electrocatalytic HER, and it is found that the octahedral Cu2 O nanostructures with exposed crystal planes of (111) (O-Cu2 O) has the best hydrogen evolution performance. Operando Raman spectroscopy and ex-situ characterization techniques showed that Cu2 O is reduced during HER, in which Cu dendrites are grown on the surface of the Cu2 O nanostructures, resulting in the better HER performance of O-Cu2 O after HER (O-Cu2 O-A) compared with that of the as-prepared O-Cu2 O. Under illumination, the onset potential of O-Cu2 O-A is ca. 52 mV positive than that of O-Cu2 O, which is induced by the plasmon-activated electrochemical system consisting of Cu2 O and the in-situ generated Cu dendrites. Incident photon-to-current efficiency (IPCE) measurements and the simulated UV-Vis spectrum demonstrate the hot electron injection (HEI) from Cu dendrites to Cu2 O. Ab initio nonadiabatic molecular dynamics (NAMD) simulations revealed the transfer of photogenerated electrons (27 fs) from Cu dendrites to Cu2 O nanostructures is faster than electron relaxation (170 fs), enhancing its surface plasmons activity, and the HEI of Cu dendrites increases the charge density of Cu2 O. These make the energy level of the catalyst be closer to that of H+ /H2 , evidenced by the plasmon-enhanced HER electrocatalytic activity.

Defect-regulated charge carrier dynamics in two-dimensional ZnO/MoS2 heterostructure

Van der Waals ZnO/MoS2 heterostructure has been experimentally demonstrated as one of the potential candidates for photocatalyst, however, the charge carrier dynamics upon photoexcitation still remains unclear. By using nonadiabatic molecular dynamics simulations, we mainly focus on the influences of interfacial point defects on photogenerated charge separation in the ZnO/MoS2. The results reveal that oxygen vacancy in ZnO layer can induce a higher hole transfer efficiency compared to the pristine ZnO/MoS2, which attributes to the enhanced nonadiabatic coupling, originating from an out-of-plane vibration mode of S atoms, a decreased energy gap for intralayer hole transfer and stronger energy state oscillation. Alternatively, S vacancy in MoS2 introducing additional energy states in the band gap of ZnO/MoS2, serves as charge carrier recombination channels, and significantly reduces charge carrier lifetime, while doping O atom in S vacancy can compensate this effect. This study provides helpful guidance to design functional devices for solar energy photovoltaic conversion, based on two-dimensional ZnO/MoS2 heterostructures.

Ultrafast internal conversion and photochromism in gas-phase salicylideneaniline.

Salicylideneaniline (SA) is an archetypal system for excited-state intramolecular proton transfer (ESIPT) in non-planar systems. Multiple channels for relaxation involving both the keto and enol forms have been proposed after excitation to S1 with near-UV light. Here, we present transient absorption measurements of hot gas-phase SA, jet-cooled SA, and SA in Ar clusters using cavity-enhanced transient absorption spectroscopy (CE-TAS). Assignment of the spectra is aided by simulated TAS spectra, computed by applying time-dependent complete active space configuration interaction (TD-CASCI) to structures drawn from nonadiabatic molecular dynamics simulations. We find prompt ESIPT in all conditions followed by the rapid generation of the trans keto metastable photochrome state and fluorescent keto state in parallel. Increasing the internal energy increases the photochrome yield and decreases the fluorescent yield and fluorescent state lifetime observed in TAS. In Ar clusters, internal conversion of SA is severely hindered, but the photochrome yield is unchanged. Taken together, these results are consistent with the photochrome being produced via the vibrationally excited keto population after ESIPT.

Ultrafast Photocontrolled Rotation in a Molecular Motor Investigated by Machine Learning-Based Nonadiabatic Dynamics Simulations.

The thermal helix inversion (THI) of the overcrowded alkene-based molecular motors determines the speed of the unidirectional rotation due to the high reaction barrier in the ground state, in comparison with the ultrafast photoreaction process. Recently, a phosphine-based motor has achieved all-photochemical rotation experimentally, promising to be controlled without a thermal step. However, the mechanism of this photochemical reaction has not yet been fully revealed. The comprehensive computational studies on photoisomerization still resort to nonadiabatic molecular dynamics (NAMD) simulations based on electronic structure calculations, which remains a high computational cost for large systems such as molecular motors. Machine learning (ML) has become an accelerating tool in NAMD simulations recently, where excited-state potential energy surfaces (PESs) are constructed analytically with high accuracy, providing an efficient approach for simulations in photochemistry. Herein the reaction pathway is explored by a spin-flip time-dependent density functional theory (SF-TDDFT) approach in combination with ML-based NAMD simulations. According to our computational simulations, we notice that one of the key factors of fulfilling all-photochemical rotation in the phosphine-based motor is that the excitation energies of four isomers are similar. Additionally, a shortcut photoinduced transformation between unstable isomers replaces the THI step, which shares the conical intersection (CI) with photoisomerization. In this study, we provide a practical approach to speed up the NAMD simulations in photochemical reactions for a large system that could be extended to other complex systems.

How the Stacking Pattern Influences the Charge Transfer Dynamics of van der Waals Heterostructures: An Answer from a Time-Domain Ab Initio Study.

Here, we perform a time domain density functional study in conjunction with a non-adiabatic molecular dynamics (NAMD) simulation to investigate the charge carrier dynamics in a series of van der Waals heterostructures made of two-dimensional (2D) SnX2 (X = S or Se)-supported ZrS2, ZrSe2, and ZrSSe monolayers. Results from NAMD simulation reveal delayed electron-hole recombination (in the range of 0.53-2.13 ns) and ultrafast electron/hole transfer processes (electron transfer within 108.3-321.5 fs and hole transfer between 107.6 and 258.8 fs). The most interesting finding of our study is that switching from AB to AA stacking in the heterostructures extends the carrier lifespan by a significant amount. The delayed electron-hole recombination because of the switching stacking pattern can be rationalized by weak electron-phonon coupling, lower non-adiabatic coupling (NAC), and fast decoherence time. Thus, these insightful NAMD studies of excited charge carriers reveal that the stacking pattern variation is an effective tool to develop efficient photovoltaic devices based on 2D van der Waals heterostructures.

Zeno and Anti-Zeno Effects in Nonadiabatic Molecular Dynamics.

Decoherence plays an important role in nonadiabatic (NA) molecular dynamics (MD) simulations because it provides a physical mechanism for trajectory hopping and can alter transition rates by orders of magnitude. Generally, decoherence effects slow quantum transitions, as exemplified by the quantum Zeno effect: in the limit of infinitely fast decoherence, the transitions stop. If the measurements are not sufficiently frequent, an opposite quantum anti-Zeno effect occurs, in which the transitions are accelerated with faster decoherence. Using two common NA-MD approaches, fewest switches surface hopping and decoherence-induced surface hopping, combined with analytic examination, we demonstrate that including decoherence into NA-MD slows down NA transitions; however, many realistic systems operate in the anti-Zeno regime. Therefore, it is important that NA-MD methods describe both Zeno and anti-Zeno effects. Numerical simulations of charge trapping and relaxation in graphitic carbon nitride suggest that time-dependent NA Hamiltonians encountered in realistic systems produce robust results with respect to errors in the decoherence time, a favorable feature for NA-MD simulations.

Deciphering the Influence of Ground-State Distributions on the Calculation of Photolysis Observables.

Nonadiabatic molecular dynamics offers a powerful tool for studying the photochemistry of molecular systems. Key to any nonadiabatic molecular dynamics simulation is the definition of its initial conditions (ICs), ideally representing the initial molecular quantum state of the system of interest. In this work, we provide a detailed analysis of how ICs may influence the calculation of experimental observables by focusing on the photochemistry of methylhydroperoxide (MHP), the simplest and most abundant organic peroxide in our atmosphere. We investigate the outcome of trajectory surface hopping simulations for distinct sets of ICs sampled from different approximate quantum distributions, namely harmonic Wigner functions and ab initio molecular dynamics using a quantum thermostat (QT). Calculating photoabsorption cross-sections, quantum yields, and translational kinetic energy maps from the results of these simulations reveals the significant effect of the ICs, in particular when low-frequency (∼ a few hundred cm-1) normal modes are connected to the photophysics of the molecule. Overall, our results indicate that sampling ICs from ab initio molecular dynamics using a QT is preferable for flexible molecules with photoactive low-frequency modes. From a photochemical perspective, our nonadiabatic dynamics simulations offer an explanation for a low-energy tail observed at high excitation energy in the translational kinetic energy map of MHP.

Computational study of the adamantane cation: simulations of spectroscopy, fragmentation dynamics, and internal conversion

Diamondoids, of which adamantane (C_{10}H_{16}) is the simplest representative, constitute an intriguing class of carbon based nanomaterials with interesting chemical, mechanical and opto-electronic properties. While neutral diamondoids have been extensively studied for decades, their cationic counterparts were a subject of recent experimental investigations motivated by their potential role in astrochemistry. Here, we perform a computational study of the adamantane cation (C_{10}H_{16}^+) complementing the recent experimental findings. Specifically, we extend earlier theoretical work on vibrationally resolved electronic spectroscopy by accounting for the higher lying electronically excited states of the cation in the absorption and photoelectron spectra. We also perform adiabatic and nonadiabatic (surface hopping) molecular dynamics simulations to study (fast) fragmentation processes and electronic relaxation of C_{10}H_{16}^+. Our simulations reveal that after excitation with near-infrared–ultraviolet photons, the adamantane cation undergoes an ultrafast internal conversion to the ground (doublet) state (on a time scale of 10–100 fs depending on initial excitation energy) which can be followed by a fast fragmentation, predominantly H loss. The yield of the ultrafast hydrogen dissociation as a function of excitation energy is reported.

Modeling Charge Transport in Organic Semiconductors Using Neural Network Based Hamiltonians and Forces.

The fewest switches surface hopping method has been widely used for the simulation of charge transport in organic semiconductors. In the present study, we perform nonadiabatic molecular dynamics (NAMD) simulations of hole transport in anthracene and pentacene. The simulations employ neural network (NN) based Hamiltonians in two different nuclear relaxation schemes, which utilize either a precalculated reorganization energy or site energy gradients additionally obtained from NN models. The performance of the NN models is evaluated in reproducing hole mobilities and inverse participation ratios in terms of both quality and computational cost. The results show that charge mobilities and inverse participation ratios obtained by models, which were trained on DFTB or DFT training data, are in very good agreement with the respective QM reference method for implicit relaxation and, where available, also for explicit relaxation. Reasonable agreement with experimental hole mobilities is achieved. Utilizing our models in NAMD simulations of charge transfer amounts to a reduction of the computational cost in a range of 1 to 7 orders of magnitude compared to DFTB and DFT. This proves neural networks as promising tools for the improvement of accuracy and efficiency of charge and potentially also exciton transport simulations in complex and large molecular systems.

Theoretical analysis of the coordination-state dependency of the excited-state properties and ultrafast relaxation dynamics of bacteriochlorophyll a

The coordination-state dependency of the excited-state properties and ultrafast relaxation dynamics from the S2 to the S1 state of bacteriochlorophyll a were analyzed using quantum chemical calculations and nonadiabatic molecular dynamics simulations. TDDFT calculations and orbital analysis clarified the molecular mechanism of the decrease in the excitation energy difference caused by ligand coordination. The results of dynamics simulations also showed that the ligand accelerated the relaxation, and the estimated time constant was in quantitative agreement with the experimental data. We demonstrated the importance of the environmental effects for the relaxation dynamics, which have not been investigated theoretically.

Rationalizing the environment-dependent photophysical behavior of a DNA luminescent probe by classical and non-adiabatic molecular dynamics simulations.

Environment-sensitive fluorescent nucleoside analogs are of utmost importance to investigate the structure of nucleic acids, their intrinsic flexibility, and sequence-specific DNA- and RNA-binding proteins. The latter play indeed a key role in transcription, translation as well as in the regulation of RNA stability, localization and turnover, and many other cellular processes. The sensitivity of the embedded fluorophore to polarity, hydration, and base stacking is clearly dependent on the specific excited-state relaxation mechanism and can be rationalized combining experimental and computational techniques. In this work, we elucidate the mechanisms leading to the population of the triplet state manifold for a versatile nucleobase surrogate, namely the 2-thienyl-3-hydroxychromone in gas phase, owing to non-adiabatic molecular dynamics simulations. Furthermore, we analyze its behavior in the B-DNA environment via classical molecular dynamics simulations, which evidence a rapid extrusion of the adenine facing the 2-thienyl-3-hydroxychromone nucleobase surrogate. Our simulations provide new insights into the dynamics of this family of chromophores, which could give rise to an integrated view and a fine tuning of their photochemistry, and namely the role of excited-state intramolecular proton transfer for the rational design of the next generation of fluorescent nucleoside analogs.

Vibrational Funnels for Energy Transfer in Organic Chromophores

Photoinduced intramolecular energy transfers in multichromophoric molecules involve nonadiabatic vibronic channels that act as energy transfer funnels. They commonly take place through specific directions of motion dictated by the nonadiabatic coupling vectors. Vibrational funnels may support persistent coherences between electronic states and sometimes delineate the presence of minor alternative energy transfer pathways. The ultimate confirmation of their role on the interchromophoric energy transfer can be achieved by performing nonadiabatic excited-state molecular dynamics simulations by selectively freezing the nuclear motions in question. Our results point out this strategy as a useful tool to identify and evaluate the impact of these vibrational funnels on the energy transfer processes and guide the in silico design of materials with tunable properties and enhanced functionalities. Our work encourages applications of this methodology to different chemical and biochemical processes such as reactive scattering and protein conformational changes, to name a few.

Toward a Rigorous Theoretical Description of Photocatalysis Using Realistic Models.

This Perspective aims at providing a road map to computational heterogeneous photocatalysis highlighting the knowledge needed to boost the design of efficient photocatalysts. A plausible computational framework is suggested focusing on static and dynamic properties of the relevant excited states as well of the involved chemistry for the reactions of interest. This road map calls for explicitly exploring the nature of the charge carriers, the excited-state potential energy surface, and its time evolution. Excited-state descriptors are introduced to locate and characterize the electrons and holes generated upon excitation. Nonadiabatic molecular dynamics simulations are proposed as a convenient tool to describe the time evolution of the photogenerated species and their propagation through the crystalline structure of photoactive material, ultimately providing information about the charge carrier lifetime. Finally, it is claimed that a detailed understanding of the mechanisms of heterogeneously photocatalyzed reactions demands the analysis of the excited-state potential energy surface.

Multiconfigurational Calculations and Photodynamics Describe Norbornadiene Photochemistry.

Storing solar energy is a vital component of using renewable energy sources to meet the growing demands of the global energy economy. Molecular solar thermal (MOST) energy storage is a promising means to store solar energy with on-demand energy release. The light-induced isomerization reaction of norbornadiene (NBD) to quadricyclane (QC) is of great interest because of the generally high energy storage density (0.97 MJ kg-1) and long thermal reversion lifetime (t1/2,300K = 8346 years). However, the mechanistic details of the ultrafast excited-state [2 + 2]-cycloaddition are largely unknown due to the limitations of experimental techniques in resolving accurate excited-state molecular structures. We now present a full computational study on the excited-state deactivation mechanism of NBD and its dimethyl dicyano derivative (DMDCNBD) in the gas phase. Our multiconfigurational calculations and nonadiabatic molecular dynamics simulations have enumerated the possible pathways with 557 S2 trajectories of NBD for 500 fs and 492 S1 trajectories of DMDCNBD for 800 fs. The simulations predicted the S2 and S1 lifetimes of NBD (62 and 221 fs, respectively) and the S1 lifetime of DMDCNBD (190 fs). The predicted quantum yields of QC and DCQC are 10 and 43%, respectively. Our simulations also show the mechanisms of forming other possible reaction products and their quantum yields.

Excitation-Wavelength-Dependent Charge-Carrier Lifetime in Hematite: An Insight from Nonadiabatic Molecular Dynamics.

Experiments have reported that the photoexcited carrier lifetime in α-Fe2O3 has a significant excitation-wavelength dependence but leave the physical mechanism unresolved. In this work, we rationalize the puzzling excitation-wavelength dependence of the photoexcited carrier dynamics in Fe2O3 by performing nonadiabatic molecular dynamics simulation based on the strongly constrained and appropriately normed functional, which accurately describes the electronic structure of Fe2O3. Photogenerated electrons with lower-energy excitation relax fast in the t2g conduction band within about 100 fs, while the photogenerated electrons with higher-energy excitation undergo first a slower interband relaxation from the eg lower state to the t2g upper state on a time scale of 135 ps, followed by the much faster t2g intraband relaxation. This study provides insight into the experimentally reported excitation-wavelength dependence of the carrier lifetime in Fe2O3 and a reference for regulating photogenerated carrier dynamics in transition-metal oxides through the light excitation wavelength.

Control of excited state charge transfer dynamics of DMABN in deep eutectic solvent: Involvement of the partially twisted intermediate state

The photo-driven charge transfer process of 4-(dimethylamino) benzonitrile (DMABN) in deep eutectic solvent (DES) is investigated through steady state and time resolved spectroscopy along with non-adiabatic molecular dynamic simulations. Simultaneous coexistence of polar and non-polar domains in the DES manifests itself in different optical response and charge transfer dynamics of DMABN. The weaker correlation between solvent viscosity and observed reaction dynamics reveals the inadequacy of global macroscopic variables to rationalise the process at the microscopic level. On the other hand, ab initio computational results clearly show the presence of a distribution of partially twisted conformations, that helps to rationalise the experimental observations.