Time-dependent Configuration Interaction Singles Research Articles

WFOT: A Wave Function Overlap Tool between Single- and Multi-Reference Electronic Structure Methods for Spectroscopy Simulation.

We report the development of a novel diagnostic tool, named wave function overlap tool (WFOT), designed to evaluate the overlap between wave functions computed at single-reference [i.e., time-dependent density functional theory or configuration interaction singles (CIS)] and multireference (i.e., CASSCF/CASPT2) electronic structure levels of theory. It relies on truncating the single- and multireference WFs to CIS-like expansions spanning the same configurational space and maximizing the molecular orbital overlap by means of a unitary transformation. To demonstrate the functionality of the tool, we calculate the transient spectrum of acetylacetone by evaluating excited state absorption signals with multireference quality on top of single-reference on-the-fly dynamics simulations. Semiautomatic spectra generation is facilitated by interfacing the tool with the COBRAMM package, which also allows one to use WFOT with several quantum chemistry codes such as Gaussian, NWChem, and OpenMolcas. Other exciting possibilities for the utilization of the code beyond the simulation of transient absorption spectroscopy are eventually discussed.

Thomas–Reiche–Kuhn correction for truncated configuration-interaction spaces: Case of laser-assisted dynamical interference

The Thomas--Reiche--Kuhn sum rule is used to form an effective potential that is added to the time-dependent configuration-interaction singles (TDCIS) equations of motion in velocity gauge. The purpose of the effective potential is to include virtual coupling from singles to doubles, which is required for size-consistent velocity-gauge TDCIS results. The proposed method is compared to length-gauge TDCIS results for laser-assisted photoionization. Finally, a dynamical interference effect controlled by two-color fields is predicted for atomic targets.

General time-dependent configuration-interaction singles. II. Atomic case

We present a specialization of the grid-based implementation of the time-dependent configuration-interaction singles described in the preceding paper (arXiv:2204.09966) to the case of spherical symmetry. We describe the intricate time propagator in detail, and conclude with a few example calculations. Among these, of note are high-resolution photoelectron spectra in the vicinity of the Fano resonances in photoionization of neon, and spin-polarized photoelectrons from xenon, in agreement with recent experiments.

General time-dependent configuration-interaction singles. I. Molecular case

We present a grid-based implementation of the time-dependent configuration-interaction singles method suitable for computing the strong-field ionization of small gas-phase molecules. After outlining the general equations of motion used in our treatment of this method, we present example calculations of strong-field ionization of helium, lithium hydride, water, and ethylene that demonstrate the utility of our implementation. The following companion paper specializes to the case of spherical symmetry, which is applied to various atoms.

Effects of electronic correlation on the high harmonic generation in helium: A time-dependent configuration interaction singles vs time-dependent full configuration interaction study.

In this paper, we investigate the effects of full electronic correlation on high harmonic generation in the helium atom subjected to laser pulses of extremely high intensity. To do this, we perform real-time propagations of helium atom wavefunction using quantum chemistry methods coupled to Gaussian basis sets. Calculations are performed within the real-time time-dependent configuration interaction framework at two levels of theory: time-dependent configuration interaction with single excitations (uncorrelated method) and time-dependent full configuration interaction (fully correlated method). The electronic wavefunction is expanded in Dunning basis sets supplemented with functions adapted to describing highly excited and continuum states. We also compare the time-dependent configuration interaction results with grid-based propagations of the helium atom within the single-active-electron approximation. Our results show that when including the dynamical electron correlation, a noticeable improvement to the description of high harmonic generation (HHG) can be achieved in terms of, e.g., a more constant intensity in the lower energy part of the harmonic plateau. However, such effects can be captured only if the basis set used suffices to reproduce the most basic features, such as the HHG cutoff position, at the uncorrelated level of theory.

Relativistic time-dependent configuration-interaction singles method

In this work, a derivation and implementation of the relativistic time-dependent configuration interaction singles (RTDCIS) method is presented. Various observables for krypton and xenon atoms obtained by RTDCIS are compared with experimental data and alternative relativistic calculations. This includes energies of occupied orbitals in the Dirac-Fock ground state, Rydberg state energies, Fano resonances and photoionization cross sections. Diagrammatic many-body perturbation theory, based on the relativistic random phase approximation, is used as a benchmark with excellent agreement between RTDCIS reported at the Tamm-Dancoff level. Results from RTDCIS are computed in the length gauge, here the negative energy states can be omitted with acceptable loss of accuracy. A complex absorbing potential, that is used to remove photoelectrons far from the ion, is implemented as a scalar potential and validated for RTDCIS. The RTDCIS methodology presented here opens for future studies of strong-field processes, such as attosecond transient absorption and high-order harmonic generation, with electron and hole spin dynamics and other relativistic effects described by first principle via the Dirac equation.

Gaussian-Type Orbital Calculations for High Harmonic Generation in Vibrating Molecules: Benchmarks for H2.

The response of the hydrogen molecular ion, H2+, to few-cycle laser pulses of different intensities is simulated. To treat the coupled electron-nuclear motion, we use adiabatic potentials computed with Gaussian-type basis sets together with a heuristic ionization model for the electron and a grid representation for the nuclei. Using this mixed-basis approach, the time-dependent Schrödinger equation is solved, either within the Born-Oppenheimer approximation or with nonadiabatic couplings included. The dipole response spectra are compared to all-grid-based solutions for the three-body problem, which we take as a reference to benchmark the Gaussian-type basis set approaches. Also, calculations employing the fixed-nuclei approximation are performed, to quantify effects due to nuclear motion. For low intensities and small ionization probabilities, we get excellent agreement of the dynamics using Gaussian-type basis sets with the all-grid solutions. Our investigations suggest that high harmonic generation (HHG) and high-frequency response, in general, can be reliably modeled using Gaussian-type basis sets for the electrons for not too high harmonics. Further, nuclear motion destroys electronic coherences in the response spectra even on the time scale of about 30 fs and affects HHG intensities, which reflect the electron dynamics occurring on the attosecond time scale. For the present system, non-Born-Oppenheimer effects are small. The Gaussian-based, nonadiabatically coupled, time-dependent multisurface approach to treat quantum electron-nuclear motion beyond the non-Born-Oppenheimer approximation can be easily extended to approximate wavefunction methods, such as time-dependent configuration interaction singles (TD-CIS), for systems where no benchmarks are available.

Propensity rules and interference effects in laser-assisted photoionization of helium and neon

We investigate the angle-resolved photoelectron spectra from laser-assisted photoionization for helium and neon atoms using an ab initio method based on time-dependent surface flux and configuration interaction singles. We find that the shape of the distributions can be interpreted using a propensity rule, an intrinsic difference in the absorption and the emission processes, as well as interference effects between multiple paths to the final angular momentum state. The difference between absorption and emission is hidden in the first sideband in neon due to the multiple competing m channels, while in the second sideband a qualitative difference between absorption and emission is observed. In the m-resolved case, a clear difference is observed already in the first sideband, as exemplified in helium and m-resolved neon.

Gauge-invariant time-dependent configuration interaction singles method: applications to high-harmonic generation from atoms

SynopsisWe present gauge-invariant reformulation of time-dependent configuration interaction singles (TDCIS) method and its applications to high-harmonic generation (HHG) from three-dimensional atoms. The conventional TDCIS method suffers from a violation of gauge-invariance, which prevents efficient simulations of high-field phenomena within the velocity gauge. In our reformulation, rotated velocity gauge transformed from the length gauge is employed instead of the velocity gauge. We simulate HHG from atoms and show gauge-invariance and performance of our rotated velocity gauge.

Preparation and Characterization of the β-Cyclodextrin Inclusion Complex with Benzbromarone

Benzbromarone (BZB) is a drug that has diuretic activity and is used in the gout treatment. Its therapeutic efficiency is decreased by its low water solubility (11.8 mg L-1) and highly hydrophobic aspect (logP 2.7), which are responsible for affecting its intestinal absorption and bioavailability. BZB inclusion complex (IC) aims at increasing the drug solubility in water and reduce adverse effects resulting in more efficient formulations. The coprecipitation encapsulating method was used to obtain the IC of BZB in β-cyclodextrin (β-CD) and the complex (BZB@β-CD) was characterized via physical-chemical analysis. A comparative study of time-dependent density functional theory (TD-DFT) and configuration interaction singles (CIS), along with “our own N-layered integrated molecular orbital and molecular mechanics” (ONIOM) method, has been carried out on the UV-Vis absorption of BZB and BZB@β-CD. The qualitative description of the simulated spectrum, given by ONIOM(CIS:PM6), is in accord with the formation of BZB@β-CD. It only became possible when phosphate buffer (pH 7.4) was used to prepare the solutions, since the complex formation did not occur in deionized water or buffered acid. Furthermore, by using the Job plot, the Scatchard method and fluorescence, it was possible to conclude that the complex molecular stoichiometry was 1:1 (1 BZB to 1 β-CD).

Implementation of a gauge-invariant time-dependent configuration-interaction-singles method for three-dimensional atoms

We present a numerical implementation of the gauge-invariant time-dependent configuration interaction singles (TDCIS) method [Appl. Sci. 8, 433 (2018)] for three-dimensional atoms. In our implementation, orbital-like quantity called channel orbital [Phys. Rev. A 74, 043420 (2006)] is propagated instead of configuration-interaction (CI) coefficients, which removes a computational bottleneck of explicitly calculating and storing numerous virtual orbitals. Furthermore, besides its physical consistency, the gauge-invariant formulation allows to take advantages of the velocity gauge treatment of the laser-electron interaction over the length gauge one in the simulation of high-field phenomena. We apply the present implementation to high-harmonic generation from helium and neon atoms, and numerically confirms the gauge invariance and demonstrates the effectiveness of the rotated velocity gauge treatment.

Discriminating organic isomers by high harmonic generation: A time-dependent configuration interaction singles study.

High Harmonic Generation (HHG) is a nonlinear optical process that provides a tunable source for high-energy photons and ultrashort laser pulses. Recent experiments demonstrated that HHG spectroscopy may also be used as an analytical tool to discriminate between randomly oriented configurational isomers of polyatomic organic molecules, namely, between the cis- and trans-forms of 1,2-dichloroethene (DCE) [M. C. H. Wong et al., Phys. Rev. A 84, 051403 (2011)]. Here, we suggest as an economic and at the same time a reasonably accurate method to compute HHG spectra for polyatomic species, Time-Dependent Configuration Interaction Singles (TD-CIS) theory in combination with extended atomic orbital bases and different models to account for ionization losses. The HHG spectra are computed for aligned and unaligned cis- and trans-DCE. For the unaligned case, a coherent averaging over possible rotational orientations is introduced. Furthermore, using TD-CIS, possible differences between the HHG spectra of cis- and trans-DCE are studied. For aligned molecules, spectral differences between cis and trans emerge, which can be related to their different point group symmetries. For unaligned, randomly oriented molecules, we also find distinct HHG spectra in partial agreement with experiment. In addition to HHG response in the frequency space, we compute time-frequency HHG spectra to gain insight into which harmonics are emitted at which time. Further differences between the two isomers emerge, suggesting time-frequency HHG as another tool to discriminate configurational isomers.

HHG probing of atomic dipoles by electronic wave-packet caustics

We exploit high-order harmonic generation spectroscopy at the caustics of the recombining electron wave-packet as a method for directly comparing experimental spectra with ab-initio theories. Experimental results in xenon and comparison with ab-initio time-dependent configuration-interaction singles calculations allowed to assess the role of the wave-packet enhancement during the giant resonance. Results in argon show that this technique can also be applied to other targets.

High-order harmonic generation spectroscopy by recolliding electron caustics

Spectral focusing of the recolliding electron in high-order harmonic generation driven by two-color fields is shown to be a powerful tool for isolating and enhancing hidden spectral features of the target under study. In previous works we used this technique for probing multi-electron effects in xenon and we compared our experimental results with time-dependent configuration-interaction singles calculations. We demonstrate here that this technique can be exploited for reconstructing the enhancement factor of the xenon giant dipole resonance and we discuss the sensitivity of this method to macroscopic effects. We then extend the technique to argon in order to test the applicability of this procedure to other targets.

Gauge-Invariant Formulation of Time-Dependent Configuration Interaction Singles Method

We propose a gauge-invariant formulation of the channel orbital-based time-dependent configuration interaction singles (TDCIS) method [Phys. Rev. A, 74, 043420 (2006)], one of the powerful ab initio methods to investigate electron dynamics in atoms and molecules subject to an external laser field. In the present formulation, we derive the equations of motion (EOMs) in the velocity gauge using gauge-transformed time-dependent, not fixed, orbitals that are equivalent to the conventional EOMs in the length gauge using fixed orbitals. The new velocity-gauge EOMs avoid the use of the length-gauge dipole operator, which diverges at large distance, and allows us to exploit computational advantages of the velocity-gauge treatment over the length-gauge one, e.g., a faster convergence in simulations with intense and long-wavelength lasers, and the feasibility of exterior complex scaling as an absorbing boundary. The reformulated TDCIS method is applied to an exactly solvable model of one-dimensional helium atom in an intense laser field to numerically demonstrate the gauge invariance. We also discuss the consistent method for evaluating the time derivative of an observable, which is relevant, e.g., in simulating high-harmonic generation.

Absorption Spectra for Disordered Aggregates of Chromophores Using the Exciton Model.

Optimizing the optical properties of large chromophore aggregates and molecular solids for applications in photovoltaics and nonlinear optics is an outstanding challenge. It requires efficient and reliable computational models that must be validated against accurate theoretical methods. We show that linear absorption spectra calculated using the molecular exciton model agree well with spectra calculated using time-dependent density functional theory and configuration interaction singles for aggregates of strongly polar chromophores. Similar agreement is obtained for a hybrid functional (B3LYP), a long-range corrected hybrid functional (ωB97X), and configuration interaction singles. Accounting for the electrostatic environment of individual chromophores in the parametrization of the exciton model with the inclusion of atomic point charges significantly improves the agreement of the resulting spectra with those calculated using all-electron methods; different charge definitions (Mulliken and ChelpG) yield similar results. We find that there is a size-dependent error in the exciton model compared with all-electron methods, but for aggregates with more than six chromophores, the errors change slowly with the number of chromophores in the aggregate. Our results validate the use of the molecular exciton model for predicting the absorption spectra of bulk molecular solids; its formalism also allows straightforward extension to calculations of nonlinear optical response.

Optimized pulses for Raman excitation through the continuum: Verification using the multiconfigurational time-dependent Hartree-Fock method

We have verified a mechanism for Raman excitation of atoms through continuum levels previously obtained by quantum optimal control using the multiconfigurational time-dependent Hartree-Fock (MCTDHF) method. For the optimal control, which requires running multiple propagations to determine the optimal pulse sequence, we used the computationally inexpensive time-dependent configuration interaction singles (TDCIS) method. TDCIS captures all of the necessary correlation of the desired processes but assumes that ionization pathways reached via double excitations are not present. MCTDHF includes these pathways and all multiparticle correlations in a set of time-dependent orbitals. The mechanism that was determined to be optimal in the Raman excitation of the Ne $1{s}^{2}2{s}^{2}2{p}^{5}3{p}^{1}$ valence state via the metastable $1{s}^{2}2{s}^{1}2{p}^{6}3{p}^{1}$ resonance state involves a sequential resonance-valence excitation. First, a long pump pulse excites the core-hole state, and then a shorter Stokes pulse transfers the population to the valence state. This process represents the first step in a multidimensional x-ray spectroscopy scheme that will provide a local probe of valence electronic correlations. Although at the optimal pulse intensities at the TDCIS level of theory the MCTDHF method predicts multiple ionization or excitation ionization of the atom, at slightly lower intensities (reduced by a factor of about 4) the TDCIS mechanism is shown to hold qualitatively. Quantitatively, the MCTDHF populations are reduced from the TDCIS calculations by a factor of 4.

Attosecond x-ray scattering from a particle-hole wave packet

We investigate the imaging of electron wave-packet dynamics by measuring the pattern of inelastically scattered photons. For this purpose, we develop a theory of time-resolved Compton scattering. We demonstrate that, by using a sufficiently short x-ray pulse, the scattering cross section directly reflects the instantaneous momentum density of the electron. Therefore, we propose time-resolved Compton scattering as a tool to image electron wave-packet dynamics in momentum space. To illustrate this, we simulate electron wave packets in argon by using the time-dependent configuration-interaction singles method. Specifically, we consider coherent particle-hole wave packets, where the hole is in either the $3p$ or the $3s$ shell, and the particle (the excited electron) is either in a Rydberg state or in the continuum. Our calculations confirm that the dynamics of electron wave packets can indeed be imaged by measuring the doubly differential Compton scattering cross section. When the x-ray detector has no energy resolution at all, the contribution of Compton scattering to the differential scattering cross section becomes stationary. In that case, the motion of the particle and the hole can no longer be inferred from the scattering pattern.

Attosecond dynamics of light-induced resonant hole transfer in high-order-harmonic generation

We present a study of high-order-harmonic generation (HHG) assisted by extreme ultraviolet (XUV) attosecond pulses, which can lead to the excitation of inner-shell electrons and the generation of a second HHG plateau. With the treatment of a one-dimensional model of krypton, based on time-dependent configuration interaction singles (TDCIS) of an effective two-electron system, we show that the XUV-assisted HHG spectrum reveals the duration of the semiclassical electron trajectories. The results are interpreted by the strong-field approximation (SFA) and the importance of the hole transfer during the tunneling process is emphasized. Finally, coherent population transfer between the inner and outer holes with attosecond pulse trains is discussed.

Time-dependent configuration-interaction-singles calculation of the 5p -subshell two-photon ionization cross section in xenon

The $5p$ two-photon ionization cross section of xenon in the photon-energy range below the one-photon ionization threshold is calculated within the time-dependent configuration-interaction-singles (TDCIS) method. The TDCIS calculations are compared to random-phase-approximation (RPA) calculations [Wendin \textit{et al.}, J. Opt. Soc. Am. B \textbf{4}, 833 (1987)] and are found to reproduce the energy positions of the intermediate Rydberg states reasonably well. The effect of interchannel coupling is also investigated and found to change the cross section of the $5p$ shell only slightly compared to the intrachannel case.