Real-time Time-dependent Density Functional Theory Research Articles

Efficient exact exchange using Wannier functions and other related developments in planewave-pseudopotential implementation of RT-TDDFT.

The plane-wave pseudopotential (PW-PP) formalism is widely used for the first-principles electronic structure calculation of extended periodic systems. The PW-PP approach has also been adapted for real-time time-dependent density functional theory (RT-TDDFT) to investigate time-dependent electronic dynamical phenomena. In this work, we detail recent advances in the PW-PP formalism for RT-TDDFT, particularly how maximally localized Wannier functions (MLWFs) are used to accelerate simulations using the exact exchange. We also discuss several related developments, including an anti-Hermitian correction for the time-dependent MLWFs (TD-MLWFs) when a time-dependent electric field is applied, the refinement procedure for TD-MLWFs, comparison of the velocity and length gauge approaches for applying an electric field, and elimination of long-range electrostatic interaction, as well as usage of a complex absorbing potential for modeling isolated systems when using the PW-PP formalism.

Reduction reactions at the interface between CdS quantum dot and Z-type ligands driven by electron injection in the electroluminescent processes.

The efficient and stable electroluminescence of quantum dots (QDs) is of great importance in their applications in new display technologies. The short service life of blue QDs, however, hinders their development and commercialization. Different mechanisms have been proposed for the destabilization of QDs in electroluminescent processes. Based on real-time time-dependent density functional theory studies on the QD models covered by Z-type ligands (XAc2, X = Cd, Zn, Mg), the structural evolution is simulated to reveal the mechanism of the reduction reactions induced by electron injection. Our simulations reproduce the experimental observations that the reduction reactions occur at the QD-ligand interface, and the reduced Cd atom is almost in a zero valence state. However, different sites are predicted for the reactions in which the surface metal atom of the QD instead of the metal atom in the ligands is reduced. As a result, one of the arms of the chelate ligand leaves the QD, which tends to cause damage to its electroluminescent performance. Our findings contribute to a mechanistic understanding of the reduction reactions that occurred at the QD-ligand interface.

Efficient exact exchange using Wannier functions and other related developments in planewave-pseudopotential implementation of RT-TDDFT.

The plane-wave pseudopotential (PW-PP) formalism is widely used for the first-principles electronic structure calculation of extended periodic systems. The PW-PP approach has also been adapted for real-time time-dependent density functional theory (RT-TDDFT) to investigate time-dependent electronic dynamical phenomena. In this work, we detail recent advances in the PW-PP formalism for RT-TDDFT, particularly how maximally localized Wannier functions (MLWFs) are used to accelerate simulations using the exact exchange. We also discuss several related developments, including an anti-Hermitian correction for the time-dependent MLWFs (TD-MLWFs) when a time-dependent electric field is applied, the refinement procedure for TD-MLWFs, comparison of the velocity and length gauge approaches for applying an electric field, and elimination of long-range electrostatic interaction, as well as usage of a complex absorbing potential for modeling isolated systems when using the PW-PP formalism.

Reduction reactions at the interface between CdS quantum dot and Z-type ligands driven by electron injection in the electroluminescent processes.

The efficient and stable electroluminescence of quantum dots (QDs) is of great importance in their applications in new display technologies. The short service life of blue QDs, however, hinders their development and commercialization. Different mechanisms have been proposed for the destabilization of QDs in electroluminescent processes. Based on real-time time-dependent density functional theory studies on the QD models covered by Z-type ligands (XAc2, X = Cd, Zn, Mg), the structural evolution is simulated to reveal the mechanism of the reduction reactions induced by electron injection. Our simulations reproduce the experimental observations that the reduction reactions occur at the QD-ligand interface, and the reduced Cd atom is almost in a zero valence state. However, different sites are predicted for the reactions in which the surface metal atom of the QD instead of the metal atom in the ligands is reduced. As a result, one of the arms of the chelate ligand leaves the QD, which tends to cause damage to its electroluminescent performance. Our findings contribute to a mechanistic understanding of the reduction reactions that occurred at the QD-ligand interface.

Ultrafast charge carrier dynamics of methylammonium lead iodide from first principles

Methylammonium lead iodide (MAPbI3) has been a major focus of photovoltaic research for the last decade. The unique interplay between the structural and electronic properties of this material contributes to its exciting optical properties especially under the action of an ultrafast laser pulse. First-principles methods like real-time time-dependent density functional theory (RT-TDDFT) enable performing corresponding simulations without the aid of empirical parameters: the gained knowledge can be applied to future studies of other complex materials. In this work, we investigate the ultrafast charge-carrier dynamics and the nonlinear optical response of MAPbI3 excited by a resonant pulse above the gap. First, we examine the electronic and optical properties in the static regime. Next, we impinge the system with a femtosecond field of varying intensity and follow the evolution of the photoexcited carrier density. A pronounced intensity-dependent response is observed, manifested by high-harmonic generation and nonlinear trends in the number of excited electrons and excitation energy. Our results provide relevant indications about the behavior of MAPbI3 under strong and coherent radiation and confirm that RT-TDDFT is a viable tool to simulate the photo-induced dynamics of complex materials from first principles.

A theoretical study on the electronic and optical properties of M-TiO2/ZnO (M=Li, Na, K, Fe) toward application in photocatalysis

This study employs the tight-binding density functional method GFN1-xTB to investigate the structural and electronic properties of TiO2-ZnO and TiO2-ZnO composite materials modified by Li, Na, K, and Fe metals. Computational analyses reveal the formation of weak covalent bonds between the TiO2 and ZnO components within the composite system. Doping TiO2-ZnO with metals induces significant alterations in the electronic structure, particularly in terms of ionization energy and global electrophilic index. The UV-VIS spectra are calculated using real-time time-dependent density functional theory with xTB Hamiltonians. The obtained results demonstrate minimal impact of metal presence on the absorption spectrum of TiO2-ZnO, but significant influence on the recombination potential of photogenerated charge carriers. It is suggested that Fe/TiO2-ZnO and Li/TiO2-ZnO will demonstrate higher photocatalytic activity than the pristine TiO2-ZnO material.

Real-Time Simulation of Ultrafast Electronic Dynamics of Nanoscale Systems Involving an Organic Molecule and a Nanoparticle Dimer.

Understanding and predicting the behavior of nanomaterials composed of plasmons interacting with quantum emitters at ultrafast timescales is crucial for the better manipulation of light at the nanoscale and advancing technologies like ultrafast communication and computing. Here we perform a simulation of the "real-time" electronic dynamics of a coupled molecule-metal nanoparticle dimer interacting with an ultrashort resonant laser pulse by combining the real-time time-dependent density functional theory (RT-TDDFT) approach with the time-domain frequency-dependent fluctuating charge (TD-ωFQ) model, an atomistic electromagnetic (AEM) model for the dynamic plasmonic response of nanoparticles. It is shown that the induced dipoles evolve from an exponential decay pattern to a beat pattern with an increase in coupling strength, which is altered by changing the molecular orientation relative to the dimer axis. It is further shown that in the strong coupling regime, both the excited molecule and the plasmon relax rapidly due to the molecule-plasmon interaction, and the efficient coherent energy exchange between the interacting molecule and plasmon modes occurs on a femtosecond (fs) timescale. This work provides guidance on manipulating light-matter interaction and studying molecular plasmonics at extremely fast timescales.

Propagated (fragment) Pipek-Mezey Wannier functions in real-time time-dependent density functional theory.

Localization procedures are an important tool for analysis of complex systems in quantum chemistry, since canonical molecular orbitals are delocalized and can, therefore, be difficult to align with chemical intuition and obscure information at the local level of the system. This especially applies to calculations obeying periodic boundary conditions. The most commonly used approach to localization is Foster-Boys Wannier functions, which use a unitary transformation to jointly minimize the second moment of the orbitals. This procedure has proven to be robust and fast but has a side effect of often mixing σ- and π-type orbitals. σ/π-separation is achieved by the Pipek-Mezey Wannier function (PMWF) approach [Lehtola and Jónsson, J. Chem. Theory Comput. 10, 642 (2014) and Jónsson et al., J. Chem. Theory Comput. 13, 460 (2017)], which defines the spread functional in terms of partial charges instead. We have implemented a PMWF algorithm in the CP2K software package using the Cardoso-Souloumiac algorithm to enable their application to real-time time-dependent density functional theory. The method is demonstrated on stacked CO2 molecules, linear acetylenic carbon, boron and nitrogen co-doped graphene, and nitrogen-vacancy doped diamond. Finally, we discuss its computational scaling and recent efforts to improve it with fragment approaches.

Time-dependent Kohn-Sham electron dynamics coupled with nonequilibrium plasmonic response via atomistic electromagnetic model.

Computational modeling of plasmon-mediated molecular photophysical and photochemical behaviors can help us better understand and tune the bound molecular properties and reactivity and make better decisions to design and control nanostructures. However, computational investigations of coupled plasmon-molecule systems are challenging due to the lack of accurate and efficient protocols to simulate these systems. Here, we present a hybrid scheme by combining the real-time time-dependent density functional theory (RT-TDDFT) approach with the time-domain frequency dependent fluctuating charge (TD-ωFQ) model. At first, we transform ωFQ in the frequency-domain, an atomistic electromagnetic model for the plasmonic response of plasmonic metal nanoparticles (PMNPs), into the time-domain and derive its equation-of-motion formulation. The TD-ωFQ introduces the nonequilibrium plasmonic response of PMNPs and atomistic interactions to the electronic excitation of the quantum mechanical (QM) region. Then, we combine TD-ωFQ with RT-TDDFT. The derived RT-TDDFT/TD-ωFQ scheme allows us to effectively simulate the plasmon-mediated "real-time" electronic dynamics and even the coupled electron-nuclear dynamics by combining them with the nuclear dynamics approaches. As a first application of the RT-TDDFT/TD-ωFQ method, we study the nonradiative decay rate and plasmon-enhanced absorption spectra of two small molecules in the proximity of sodium MNPs. Thanks to the atomistic nature of the ωFQ model, the edge effect of MNP on absorption enhancement has also been investigated and unveiled.

The cage effect of electron beam irradiation damage in cryo-electron microscopy

Electron beam irradiation can cause damage to biological and organic samples, as determined via transmission electron microscopy (TEM). Cryo-electron microscopy (cryo-EM) significantly reduces such damage by quickly freezing the environmental water around organic molecules. However, there are multiple hypotheses about the mechanism of cryo-protection in cryo-EM. A lower temperature can cause less molecular dissociation in the first stage, or frozen water can have a “cage” effect by preventing the dissociated fragments from flying away. In this work, we use real-time time-dependent density functional theory molecular dynamics(rt-TDDFT-MD) simulations to study the related dynamics. We use our recently developed natural orbital branching (NOB) algorithm to describe the molecular dissociation process after the molecule is ionized. We find that despite the difference in surrounding water molecules at different temperatures, the initial dissociation process is similar. On the other hand, the dissociated fragments fly away at room temperature, while they remain in the same cage when frozen water is used. Our results provide direct support for the cage effect mechanism.

Optical-Helicity-Dependent Orbital and Spin Dynamics in Two-Dimensional Ferromagnets.

Disentangling orbital (OAM) and spin (SAM) angular momenta in the ultrafast spin dynamics of two-dimensional (2D) ferromagnets on subfemtoseconds is a challenge in the field of ultrafast magnetism. Herein, we employed a non-collinear spin version of real-time time-dependent density functional theory to investigate the orbital and spin dynamics of the 2D ferromagnets Fe3GeTe2 (FGT) induced by circularly polarized light. Our results show that the demagnetization of the Fe sublattice in FGT is accompanied by helicity-dependent precession of the OAM and SAM excited by circularly polarized lasers. We further identify that precession of the OAM and SAM in FGT is faster than demagnetization within a few femtoseconds. Remarkably, circularly polarized lasers can significantly induce a periodic transverse linear response of the OAM and SAM on very ultrafast time scales of ∼600 attoseconds. Our finding suggests a powerful new route for attosecond regimes of the angular momentum manipulation to coherently control helicity-dependent orbital and spin dynamics in 2D ferromagnets.

Localization of hot carriers in Au144(SCH3)60 clusters doped with copper

Hot carriers (HCs) generated by visible light in metal nanostructures present significant potential for various applications. Nevertheless, comprehending HCs at a microscopic level has been a formidable challenge. This research delves into the spatial distribution of HCs within thiolate-protected Au144(SCH3)60 cluster doped with copper through real-time time-dependent density functional theory (rt-TDDFT). By accurately adjusting a Gaussian laser pulse to correspond to the dominant photoabsorption frequencies, we reveal the formation of hot electrons and hot holes. Throughout all the examined configurations, it was noted that copper doping creates a more pronounced influence on the distribution of hot holes compared to hot electrons, indicating the potential for precise control over HC generation in nanoscale systems with well-defined electronic structures.

Monitoring Density Redistribution at the Excited State in a Dual Emitting Molecule: An Analysis Based on Real-Time Density Functional Theory and Density Descriptors.

In this work, we computed and analyzed, by means of density-based descriptors, the real-time evolution of both the locally excited (LE) and charge-transfer (CT) excited states for the planar and twisted conformations of the DMABN (4-(N,N-dimethylamino)benzonitrile) molecule using real-time time-dependent density functional theory (DFT) and three different exchange-correlation energy functionals (EXC) belonging to the same family (the PBE one). Our results based on the analysis of density-based descriptors show that the underlying EXC modifies the evolution in time of the density. In particular, comparing the frequency of density reorganization computed with the three functionals (PBE, PBE0, and LC-PBE), we found that the frequency of electronic interconversion of the individual determinants involved during the dynamics increases from PBE to PBE0 and to LC-PBE. This allows us to show that there is a correlation between the delocalization of the electronic density and the frequency of reorganization. In particular, the greater the mean hole-electron distance during the dynamics, the lower is the frequency of density reorganization.

Efficient computational modeling of electronic stopping power of organic polymers for proton therapy optimization

This comprehensive study delves into the intricate interplay between protons and organic polymers, offering insights into proton therapy in cancer treatment. Focusing on the influence of the spatial electron density distribution on stopping power estimates, we employed real-time time-dependent density functional theory coupled with the Penn method. Surprisingly, the assumption of electron density homogeneity in polymers is fundamentally flawed, resulting in an overestimation of stopping power values at energies below 2 MeV. Moreover, the Bragg rule application in specific compounds exhibited significant deviations from experimental data around the stopping maximum, challenging established norms.

Laser-Induced Ultrafast Spin Injection in All-Semiconductor Magnetic CrI3/WSe2 Heterobilayer.

Spin injection stands out as a crucial method employed for initializing, manipulating, and measuring the spin states of electrons, which are fundamental to the creation of qubits in quantum computing. However, ensuring efficient spin injection while maintaining compatibility with standard semiconductor processing techniques is a significant challenge. Herein, we demonstrate the capability of inducing an ultrafast spin injection into a WSe2 layer from a magnetic CrI3 layer on a femtosecond time scale, achieved through real-time time-dependent density functional theory calculations upon a laser pulse. Following the peak of the magnetic moment in the CrI3 sublayer, the magnetic moment of the WSe2 layer reaches a maximum of 0.89 μB (per unit cell containing 4 WSe2 and 1 CrI3 units). During the spin dynamics, spin-polarized excited electrons transfer from the WSe2 layer to the CrI3 layer via type-II band alignment. The large spin splitting in conduction bands and the difference in the number of spin-polarized local unoccupied states available in the CrI3 layer lead to a net spin in the WSe2 layer. Furthermore, we confirmed that the number of available states, the spin-flip process, and the laser pulse parameters play important roles during the spin injection process. This work highlights the dynamic and rapid nature of spin manipulation in layered all-semiconductor systems, offering significant implications for the development and enhancement of quantum information processing technologies.

Reproducibility of real-time time-dependent density functional theory calculations of electronic stopping power in warm dense matter

Reproducibility of real-time time-dependent density functional theory calculations of electronic stopping power in warm dense matter

Carrier injection induced degradation of nitrogen passivated SiC–SiO2 interface simulated by time-dependent density functional theory

The performance of SiC-based metal-oxide-semiconductor field-effect transistors (MOSFETs) degrades seriously after a period of continuous operation. To directly understand this issue, we conduct real-time time-dependent density functional theory (TDDFT) simulations on a series of nitrogen passivated SiC–SiO2 interfaces to monitor the interaction between carriers and interface atoms. We find that the nitrogen passivation always leaves behind two local states near the VBM, which gives a chance to the strong interaction between channel carriers and C–N bonds, and finally results in the generation of C dangling bond defects. These processes are vividly presented and confirmed by the TDDFT simulation. Additionally, the results show that the new defects are more easily formed by the passivated C cluster than the passivated Si vacancy. These studies provide physical insights into the degradation mechanisms of working SiC MOSFETs, while simultaneously demonstrating the advantage of TDDFT as a crucial tool for investigating defect generation dynamics in semiconductor devices.

Ultrafast Chiral Precession of Spin and Orbital Angular Momentum Induced by Circularly Polarized Laser Pulse in Elementary Ferromagnets.

Despite spin (SAM) and orbital (OAM) angular momentum dynamics being well-studied in demagnetization processes, their components receive less focus. Here, we utilize real-time time-dependent density functional theory (rt-TDDFT) to unveil significant x and y components of SAM and OAM induced by circularly left (σ+) and right (σ-) polarized laser pulses in ferromagnetic Fe, Co, and Ni. Our results show that the magnitude of the OAM is an order of magnitude larger than that of the SAM, highlighting a stronger optical response from the orbital degrees of freedom of electrons. Intriguingly, σ+ and σ- pulses induce chirality in the precession of SAM and OAM, respectively, with clear associations with laser frequency and duration. Finally, we demonstrate the time scale of the OAM and SAM precession occurs even earlier than that of the demagnetization process and the OISTR effect. Our results provide detailed insight into the dynamics of SAM and OAM during and shortly after a polarized laser pulse.

Optical properties of Cu, Ag, and Au nanoparticles with different sizes and shapes

AbstractThe absorption spectra of various sizes of nanoparticles of copper (Cu), silver (Ag), and gold (Au) are theoretically investigated. The density functional theory (DFT), time‐dependent DFT (TDDFT), and real‐time TDDFT are used to demonstrate how size and shape affect their optical properties and how these are evolved as the number of atoms increases. For this reason, the focus was turned on almost spherical nanoparticles cut out from the corresponding crystal structure (called 0D), elongated ones (1D), and flattened ones (2D). The nature of the observed absorption peaks is further analyzed with the help of transition contribution maps and induced density plots which help us identify the emergence of probable plasmonic resonances as the size of the nanoparticles increases.

Two-dimensional electronic spectroscopy from first principles

The recent development of multidimensional ultrafast spectroscopy techniques calls for the introduction of computational schemes that allow for the simulation of such experiments and the interpretation of the corresponding results from a microscopic point of view. In this work, we present a general and efficient first-principles scheme to compute two-dimensional electronic spectroscopy maps based on real-time time-dependent density-functional theory. The interface of this approach with the Ehrenfest scheme for molecular dynamics enables the inclusion of vibronic effects in the calculations based on a classical treatment of the nuclei. The computational complexity of the simulations is reduced by the application of numerical advances such as branching techniques, undersampling, and a novel reduced phase cycling scheme, applicable for systems with inversion symmetry. We demonstrate the effectiveness of this method by applying it to prototypical molecules such as benzene, pyridine, and pyrene. We discuss the role of the approximations that inevitably enter the adopted theoretical framework and set the stage for further extensions of the proposed method to more realistic systems.