Real-space Time-dependent Density Functional Theory Research Articles

Calculating Potential Energy Surfaces with Quantum Computers by Measuring Only the Density Along Adiabatic Transitions.

We show that chemically accurate potential energy surfaces (PESs) can be generated from quantum computers by measuring only the density along an adiabatic transition between different molecular geometries. In lieu of using phase estimation, the energy is evaluated by performing line-integration using the inverted real-space time-dependent density functional theory Kohn-Sham (KS) potential obtained from the geometry-varying densities of the full wave function. The accuracy of this method depends on the validity of the adiabatic evolution itself and the potential inversion process (which is theoretically exact but can be numerically unstable), whereas the total evolution time is the defining factor for the precision of phase estimation. We examine the method with a one-dimensional system of two electrons for both the ground and first triplet states in first quantization, as well as the ground state of three- and four-electron systems in second quantization. It is shown that few accurate measurements can be utilized to obtain chemical accuracy across the full potential energy curve, with a shorter propagation time than may be required using phase estimation for a similar accuracy. We also show that an accurate potential energy curve can be calculated by making many imprecise density measurements (using a few shots) along the time evolution and smoothing the resulting density evolution. Finally, it is important to note that the method is able to classically provide a check of its own accuracy by comparing the density resulting from a time-independent KS calculation using the inverted potential with the measured density. This can be used to determine whether longer adiabatic evolution times are required to satisfy the adiabatic theorem.

Dipole instability in molecules irradiated by XUV pulses

We study the response of small covalent molecules to XUV laser pulses. The theoretical description relies on a real-time and real-space Time-Dependent Density Functional Theory (TDDFT) approach at the level of the local density approximation complemented by an efficient self-interaction correction. We observe the development of a dipole instability well after the laser pulse has died out. We find that this instability mechanism is robust with respect to ionic motion, to a wide variety of laser characteristics and to the inclusion of incoherent correlations at the level of a relaxation time ansatz. To rule out any potential numerical effects, we use two independent computational implementations of the TDDFT approach. A comparison of the various laser parameters together with the widely used model approach consisting in an instantaneous hole excitation shows the generic character of this instability in terms of the level depletion of a deep lying electron state. An experimental verification of the phenomenon is proposed in terms of a time-resolved measurement of the photoelectron spectrum.

Nonlinear Polarization and Low-Dissipation Ultrafast Optical Switching in Phosphorene

Fundamental limits to the computation using silicon-based devices stimulate other emerging and viable alternatives. Manipulation of the electrical and optical properties with the ultrafast and intense electric fields offers one of such alternatives. Here, we study the interaction of high-intensity pulsed femtosecond laser of various intensities and carrier frequencies with a monolayer phosphorene using the real-time real-space time-dependent density functional theory. The nonlinear induced currents are entirely reversible but are phase-shifted compared to low-intensity lights within the period of incident laser. We observe optical Kerr effect associating the changes in the number of free charge carriers with the change in the refractive index of the material, which subsequently characterizes the material’s ability of dielectric switching. The transient changes in refractive index are reversible up to a certain threshold, suggesting that the properties can be switched on the timescale of an optical period,...

Self-consistent DFT+U method for real-space time-dependent density functional theory calculations

We implemented various DFT+U schemes, including the ACBN0 self-consistent density-functional version of the DFT+U method [Phys. Rev. X 5, 011006 (2015)] within the massively parallel real-space time-dependent density functional theory (TDDFT) code Octopus. We further extended the method to the case of the calculation of response functions with real-time TDDFT+U and to the description of non-collinear spin systems. The implementation is tested by investigating the ground-state and optical properties of various transition metal oxides, bulk topological insulators, and molecules. Our results are found to be in good agreement with previously published results for both the electronic band structure and structural properties. The self consistent calculated values of U and J are also in good agreement with the values commonly used in the literature. We found that the time-dependent extension of the self-consistent DFT+U method yields improved optical properties when compared to the empirical TDDFT+U scheme. This work thus opens a different theoretical framework to address the non equilibrium properties of correlated systems.

Pulse train dependence of electron dynamics during resonant femtosecond laser nonlinear ionization of a Na4 cluster

In this study, a real-time and real-space time-dependent density functional theory (TDDFT) is applied to describe nonlinear electron–photon interactions during a resonant femtosecond laser pulse train photoionization of a Na4 cluster. The effects of key pulse train parameters, such as the spatial/temporal pulse energy distribution, pulse number per train, pulse separation and pulse phase on resonant absorption, are discussed. The calculations show that the resonant effect and the nonlinear electron dynamics, including energy absorption, electron emission, dipole response and ionization probability, can be controlled by shaping the ultrafast laser pulse train.

Effects of key pulse train parameters on electron dynamics during femtosecond laser nonlinear ionization of silica

The controlled, well-characterized evolution of the amplitude envelope and carrier-frequency sweep of ultrafast laser pulses permits the measurement and control of quantum transitions on a femtosecond time scale. This opens new perspectives to manipulate/adjust/interfere with the transient localized electron dynamics, corresponding material properties and phase change mechanisms, which are critical in laser micro/nano fabrication. This study presents the first-principles calculations of nonlinear electron–photon interactions during femtosecond pulse train ablation of silica. A real-time and real-space time-dependent density functional theory (TDDFT) is used to describe the transient localized electron dynamics such as photon absorption, electron excitation and free electron distribution. The effects of key pulse train parameters (including the pulse separation, the number of pulses per train and temporal pulse energy distribution) on electron dynamics are investigated.

Nanoplasmon dynamics and field enhancement of graphene flakes by first-principles simulations

We investigated nanoplasmon dynamics in graphene flake (GF) monomers and dimers using real-time and real-space time-dependent density functional theory. By showing the characteristic features of dynamical polarizability, we verified that the two distinct peaks in the optical absorption spectra can be attributed to π and π + σ plasmons. We clearly show a significant difference between the on- and off-resonance responses of the π plasmon by demonstrating the spatial distribution of the induced charge and the electric field. We thus reveal the mechanism of plasmon-induced field enhancement near GF edges, which shows great importance of first-principles studies on molecular nanoplasmonics.

Frequency dependence of electron dynamics during femtosecond laser resonant photoionization of Li4 cluster

This study investigates the resonant effects on nonlinear photoionization of Li4 cluster under femtosecond laser irradiation. The real-time and real-space time-dependent density functional theory is applied to describe the optical linear response and nonlinear electron dynamics during the laser-material interactions. The calculations describe well the behaviors of the ionization process for both the off resonance and on resonance cases. For given laser intensities, the frequency dependence of electron ionization and energy absorption on multiphoton and/or tunnel ionization mechanisms is investigated. Theoretical results show that (1) the off resonance behaves as a classical oscillator whereas resonance leads to an internal excitation with subsequent emission of electrons after the laser termination; (2) when multiphoton ionization dominates, the frequency dependence of emitted electrons and absorbed energy coincides with trend of the spectral cross section for photon absorption; (3) while when tunnel ionization becomes significant, the aforementioned correlation gradually disappears, and the enhancement of resonant effect on photoionization is not manifest.

Interaction of electromagnetic fields and atomic clusters

In the framework of real-time real-space time-dependent density functional theory complemented with Ehrenfest molecular dynamics we have studied the response of nanostructures to intense femtosecond laser pulses. Examples of applications include laser desorption of hydrogen from graphene and Coulomb explosion of hydrocarbon molecules.

Nonlinear ionization mechanism dependence of energy absorption in diamond under femtosecond laser irradiation

We present first-principles calculations for nonlinear photoionization of diamond induced by the intense femtosecond laser field. A real-time and real-space time-dependent density functional theory with the adiabatic local-density approximation is applied to describe the laser-material interactions in the Kohn-Sham formalism with the self-interaction correction. For a certain laser wavelength, the intensity dependence of energy absorption on multiphoton and/or tunnel ionization mechanisms is investigated, where laser intensity regions vary from 1012 W/cm2 to 1016 W/cm2. In addition, the effect of laser wavelength on energy absorption at certain ionization mechanism is discussed when the Keldysh parameter is fixed. Theoretical results show that: (1) at the fixed laser wavelength, the relationship between the energy absorption and laser intensity shows a good fit of E = cMIN (N is the number of photons absorbed to free from the valence band) when multiphoton ionization dominates; (2) while when tunnel ionization becomes significant, the relationship coincides with the expression of E = cTIn (n < N).

Transient localized electron dynamics simulation during femtosecond laser tunnel ionization of diamond

A real-time and real-space time-dependent density-functional theory (TDDFT) is applied to simulate the nonlinear electron–photon interactions during femtosecond laser processing of diamond when tunnel ionization dominates. The transient localized electron dynamics including the electron excitation, energy absorption and electron density evolution are described in this Letter. In addition, the relationships among average absorbed energy, Keldysh parameter and laser intensity are revealed when the laser frequency is fixed.

Electron and ion dynamics in graphene and graphane fragments subjected to high-intensity laser pulses

In the framework of real-time real-space time-dependent density functional theory complemented with Ehrenfest molecular dynamics we have studied the response of small graphene and graphane fragments to intense femtosecond laser pulses. In particular, we have investigated how the response changes with laser pulses of different frequency (near IR, visible, and UV). The results of our simulations show that graphene has a very high immediate (i.e., within laser-pulse duration) damage threshold. They also suggest that, similar to the case of other hydrogenated surfaces, it should be possible to selectively desorb hydrogens from graphane without destroying the underlying carbon structure provided that the laser pulse parameters are properly chosen.

Modeling Fast Electron Dynamics with Real-Time Time-Dependent Density Functional Theory: Application to Small Molecules and Chromophores.

The response of matter to external fields forms the basis for a vast wealth of fundamental physical processes ranging from light harvesting to nanoscale electron transport. Accurately modeling ultrafast electron dynamics in excited systems thus offers unparalleled insight but requires an inherently nonlinear time-resolved approach. To this end, an efficient and massively parallel real-time real-space time-dependent density functional theory (RT-TDDFT) implementation in NWChem is presented. The implementation is first validated against linear-response TDDFT and experimental results for a series of molecules subjected to small electric field perturbations. Second, nonlinear excitation of green fluorescent protein is studied, which shows a blue-shift in the spectrum with increasing perturbation, as well as a saturation in absorption. Next, the charge dynamics of optically excited zinc porphyrin is presented in real time and real space, with relevance to charge injection in photovoltaic devices. Finally, intermolecular excitation in an adenine-thymine base pair is studied using the BNL range separated functional [ Baer , R. ; Neuhauser , D. Phys. Rev. Lett. 2005 , 94 , 043002 ], demonstrating the utility of a real-time approach in capturing charge transfer processes.

Near-field-induced optical force on a metal particle andC60: Real-time and real-space electron dynamics simulation

Optical forces induced by a near field are calculated for a 1-mm-sized metal particle mimicked by a jellium model and for ${\mathrm{C}}_{60}$ in the framework of real-time and real-space time-dependent density-functional theory combined with a nonuniform light-matter interaction formalism, fully taking account of multipole interaction. A highly localized near field nonuniformly polarizes these molecules. The locally induced polarization charges in the molecules are partly canceled by the screening charges. The polarization and screening charges generally contribute to the attractive and repulsive forces, respectively, and a sensible balance between these charges results in several peaks in the optical force as a function of the frequency of the near field. The resonance excitation does not necessarily maximally induce the net force, and the force exerted on the molecules strongly depends on the details of their electronic structures. The optical force is larger in the metal particle than in ${\mathrm{C}}_{60}$. We also found that the optical force depends linearly on the intensity of the near field.

TDDFT APPROACH TO PHOTOABSORPTION IN EVEN-EVEN NUCLEI

We describe the principle of the time-dependent density-functional theory (TDDFT), as well as that of the density functional theory for a localized density and for a deformed density profile. The real-time real-space TDDFT is utilized to calculate photoabsorption cross sections for even-even nuclei. Results for prolate and oblate nuclei, 24 Mg and 28 Si , reproduce basic features of experimental data.