First-Principles Approach for Coupled Quantum Dynamics of Electrons and Protons in Heterogeneous Systems

Presolvation Dynamics Preceding the Hydrated Proton Transfer in the Electrical Double Layer

Hydrated proton transfer (PT) mechanisms in a solid-liquid electric double layer (EDL) remain challenging, as the solvation structure is influenced by the operating potential. Under constant potential framework, we combined the fixed-potential method with ab initio molecular dynamics (AIMD) to simulate the PT in the EDL. Near the interface of EDL, negative potential (vs zero-charge potential) promotes the effective diffusion of the excess proton via reducing the proportion of trapping and revisiting processes of PT, but stiffens the hydrogen bond network manifesting as the restriction of water orientation and the elongation of the O···O pairs. At high negative potentials, hydrated hydrogen ions tend to form the Eigen cation (H2O)3H3O+ during presolvation processes, but this cation exhibits a higher energy barrier for PT than the pentamer (H2O)4H3O+ with a 4-fold coordinated shell. Further simulations reveal that the stiffening effect and the Eigen cation formation suppress proton conductivity near the electrode surface.

Energy conservation in real-time nuclear-electronic orbital Ehrenfest dynamics

Real-time nuclear-electronic orbital Ehrenfest (RT-NEO-Ehrenfest) dynamics methods provide a first-principles approach for describing nonadiabatic molecular processes with nuclear quantum effects. For an efficient description of proton transfer within RT-NEO-Ehrenfest dynamics, the basis function center associated with the quantum proton can be allowed to move classically. This traveling proton basis (TPB) approach effectively captures proton quantum dynamics, although its energy conservation behavior is not yet fully satisfactory. Two recently proposed TPB approaches, in principle, conserve the extended energy, which includes both the system energy and the kinetic energy associated with the proton basis function center. Herein, a thermostatted TPB approach is proposed to improve the conservation of the system energy, excluding the kinetic energy associated with the proton basis function center. In this approach, the quantum proton dynamics are modulated by dynamically rescaling the proton momentum operator to maintain the system energy conservation. With the excited-state intramolecular proton transfer of o-hydroxybenzaldehyde as an example, this approach is shown to significantly improve the system energy conservation while preserving the accuracy of the quantum proton dynamics as achieved in the original TPB approach.

Size Effect on Ultrafast Dynamics of the Photoexcited 7Be Electron in 7Be@C2n (2n = 60, 70, and 80)

The ultrafast excited-state dynamics of endohedral fullerenes are crucial in their photophysical and photochemical processes when they are employed as photovoltaic devices, photocatalytic devices, and single-molecule devices. In this study, by employing the ab initio non-adiabatic molecular dynamics simulations based on the time-dependent Kohn-Sham (TD-KS) method, we theoretically studied the size effect on ultrafast excited-state decay dynamics of the photoexcited 7Be electron in endohedral fullerenes 7Be@C2n (2n = 60, 70, and 80). These excited-state decay dynamics, which involve the charge-transfer process, occur in an ultrafast time scale of about 3 ps. The larger fullerene cage delays the excited-state decay process because the presence of significant energy gaps and phonon modes in large endohedral fullerenes slows the non-radiative electron transitions among energy levels. Those findings not only provide physical insights into the excited-state decay dynamics of confined atoms but also stimulate further research to develop efficient endohedral-fullerene-based photoelectric and photocatalytic devices.

Lagrangian formulation of nuclear-electronic orbital Ehrenfest dynamics with real-time TDDFT for extended periodic systems

We present a Lagrangian-based implementation of Ehrenfest dynamics with nuclear-electronic orbital (NEO) theory and real-time time-dependent density functional theory for extended periodic systems. In addition to a quantum dynamical treatment of electrons and selected protons, this approach allows for the classical movement of all other nuclei to be taken into account in simulations of condensed matter systems. Furthermore, we introduce a Lagrangian formulation for the traveling proton basis approach and propose new schemes to enhance its application for extended periodic systems. Validation and proof-of-principle applications are performed on electronically excited proton transfer in the o-hydroxybenzaldehyde molecule with explicit solvating water molecules. These simulations demonstrate the importance of solvation dynamics and a quantum treatment of transferring protons. This work broadens the applicability of the NEO Ehrenfest dynamics approach for studying complex heterogeneous systems in the condensed phase.

Extending Non-Perturbative Simulation Techniques for Open-Quantum Systems to Excited-State Proton Transfer and Ultrafast Non-Adiabatic Dynamics

Excited state proton transfer is an ubiquitous phenomenon in biology and chemistry, spanning from the ultrafast reactions of photobases and acids to light-driven, enzymatic catalysis and photosynthesis. However, the simulation of such dynamics involves multiple challenges, since high-dimensional, out-of-equilibrium vibronic states play a crucial role, while a fully quantum description of the proton's dissipative, real-space dynamics is also required. In this work, we extend the powerful matrix product state approach to open quantum systems (TEDOPA) to study these demanding dynamics, and also more general nonadiabatic processes that can appear in complex photochemistry subject to strong laser driving. As an illustration, we initially consider an open model of a four-level electronic system interacting with hundreds of intramolecular vibrations that drive ultrafast excited state proton transfer, as well as an explicit photonic environment that allows us to directly monitor the resulting dual fluorescence in this system. We then demonstrate how to include a continuous "reaction coordinate" of the proton transfer that allows numerically exact simulations that can be understood, visualized and interpreted in the familiar language of diabatic and adiabatic dynamics on potential surfaces, while also retaining an exact quantum treatment of dissipation and driving effects that could be used to study diverse problems in ultrafast photochemistry.

Nuclear-Electronic Orbital Time-Dependent Configuration Interaction Method

Combining real-time electronic structure with the nuclear-electronic orbital (NEO) method has enabled the simulation of complex nonadiabatic chemical processes. However, accurate descriptions of hydrogen tunneling and double excitations require multiconfigurational treatments. Herein, we develop and implement the real-time NEO time-dependent configuration interaction (NEO-TDCI) approach. Comparison to NEO-full CI calculations of absorption spectra for a molecular system shows that the NEO-TDCI approach can accurately capture the tunneling splitting associated with the electronic ground state as well as vibronic progressions corresponding to double electron-proton excitations associated with excited electronic states. Both of these features are absent from spectra obtained with single reference real-time NEO methods. Our simulations of hydrogen tunneling dynamics illustrate the oscillation of the proton density from one side to the other via a delocalized, bilobal proton wave function. These results indicate that the NEO-TDCI approach is highly suitable for studying hydrogen tunneling and other inherently multiconfigurational systems.

Real-Time Time-Dependent Density Functional Theory for Simulating Nonequilibrium Electron Dynamics

The explicit real-time propagation approach for time-dependent density functional theory (RT-TDDFT) has increasingly become a popular first-principles computational method for modeling various time-dependent electronic properties of complex chemical systems. In this Perspective, we provide a nontechnical discussion of how this first-principles simulation approach has been used to gain novel physical insights into nonequilibrium electron dynamics phenomena in recent years. Following a concise overview of the RT-TDDFT methodology from a practical standpoint, we discuss our recent studies on the electronic stopping of DNA in water and the Floquet topological phase as examples. Our discussion focuses on how RT-TDDFT simulations played a unique role in deriving new scientific understandings. We then discuss existing challenges and some new advances at the frontier of RT-TDDFT method development for studying increasingly complex dynamic phenomena and systems.