Quantum Dynamics Calculations Research Articles

A quantum wavepacket study of state-to-state photodissociation dynamics of HOBr/DOBr

Photodissociation of HOBr is an important step in the reaction network of the depletion of ozone in stratosphere. Here, we report the first three-dimensional potential energy surfaces for the lowest three singlet states for HOBr, based on high level multi reference configuration interaction calculations. Quantum dynamics calculations are performed with a real wavepacket method, yielding not only absorption spectra but also internal state and angular distributions of the photodissociation fragments. Our results agree quantitatively with the measured total absorption cross sections of HOBr in the ultraviolet region and reproduce well the observed vibrationally cold and rotationally hot OH/OD fragments via photodissociation of HOBr/DOBr at 266 nm. In addition, we predict that the recoil anisotropy parameters for OH/OD are close to the limiting value of a parallel transition, suggesting a rapid dissociation process at 266 nm following an in-plane transition from the ground state (11A′) to the 21A′ state. This is consistent with the experimental conclusion derived from the measured rotational alignment. However, spin and electronic angular momenta need to be taken into account in the future to achieve a more quantitative agreement with experiment. Our work is expected to motivate further experimental investigations for this benchmark system.

Field Programmable Gate Arrays for Enhancing the Speed and Energy Efficiency of Quantum Dynamics Simulations.

We present the first application of field programmable gate arrays (FPGAs) as new, customizable hardware architectures for carrying out fast and energy-efficient quantum dynamics simulations of large chemical/material systems. Instead of tailoring the software to fixed hardware, which is the typical case for writing quantum chemistry code for central processing units (CPUs) and graphics processing units (GPUs), FPGAs allow us to directly customize the underlying hardware (even at the level of specific electrical signals in the circuit) to give a truly optimized computational performance for quantum dynamics calculations. By offloading the most intensive and repetitive calculations onto an FPGA, we show that the computational performance of our real-time electron dynamics calculations can even exceed that of optimized commercial mathematical libraries running on high-performance GPUs. In addition to this impressive computational speedup, we show that FPGAs are immensely energy-efficient and consume 4 times less energy than modern GPU or CPU architectures. These energy savings are a practical and important metric for supercomputing centers (many of which exceed over $1 million in power costs alone), as exascale computing capabilities become more widespread and commonplace. Taken together, the implementation techniques and performance metrics of our study demonstrate that FPGAs could play a promising role in upcoming quantum chemistry and materials science applications, particularly for the acceleration and energy-efficient execution of quantum dynamics calculations.

Line-shape parameters for the first rotational lines of HD in He

We report theoretical and experimental line-shape parameters for He-perturbed pure rotational HD lines that are relevant for the studies of gas giants atmospheres. Besides the usual pressure broadening and shift parameters, we also report their speed dependencies and Dicke parameters. The theoretical values, obtained from quantum dynamical calculations, are for the R(j=0-3) lines and S(j=0-2) and temperatures from 10 to 500 K. The measurements, performed using stimulated Raman spectroscopy, were done for the S(j=0-2) rotational Raman lines at 77, 195 and 298 K. We also compare the results of our calculations with pressure broadening and line shift coefficients available in the literature at 77, 195 and 300 K for the studied R lines. We demonstrate that a simple Voigt profile is insufficient to accurately model the shapes of He-perturbed HD lines at conditions relevant to gas giants atmospheres, and one should incorporate also the speed-dependent effects and velocity-changing collisions.

Entanglement dynamics for quantum emitters strongly coupled with molybdenum disulfide nanodisks

We study the dynamics of entanglement for a composite two-qubit system, where each qubit is close to a molybdenum disulfide nanodisk, by combining quantum dynamics and classical electromagnetic calculations. We assume that each qubit interacts locally with its own photonic environment and its free-space decay time is in the picosecond regime. We observe non-Markovian entanglement dynamics indicating strong light-matter interaction between the qubits and the photonic reservoirs. We investigate the entanglement dynamics using entanglement of formation as a measure of entanglement by varying the initial state of the composite system, which always remains an extended Werner-like state, the transition frequency and the orientation of the transition dipole moment of each qubit, the radius of the molybdenum disulfide nanodisk, and the distance of a qubit from its corresponding nanodisk. For several values of the above parameters, we observe interesting phenomena during the time evolution of entanglement of formation, such as entanglement sudden death, revival of entanglement and entanglement trapping. • The entanglement dynamics of two initially entangled quantum emitters next to MoS 2 nanodisks is analyzed. • The results are obtained by combining quantum dynamics calculations with electromagnetic calculations. • Non-Markovian entanglement dynamics is obtained. • Entanglement sudden death, revival of entanglement and entanglement trapping are found for different system parameters. • The effects of the electric dipole orientation of the quantum emitter are also analyzed.

Quantum dynamics study of isotope effects of the OD/OH + CH3 reactions

ABSTRACT A six-degrees-of-freedom, time-dependent quantum dynamics calculation is employed to study the integral cross sections, full-dimensional cumulative reaction probabilities and full-dimensional rate constants for the isotopic reactions of the OD and OH with CH3 reactions. The full-dimensional cumulative reaction probabilities and full-dimensional rate constants are obtained using the energy and J-K shifting approaches based on the six-degrees-of-freedom calculations. The comparison of integral cross sections shows that the OD + CH3 reaction has a larger energy threshold and a smaller tunnelling effect than the OH + CH3 reaction. The corrected rate constants using the experimental zero-point energy have a very good agreement with the experimental results. The comparison of the rate constants shows that the OD + CH3 has smaller rate constants than the OH + CH3 reaction, which indicates a smaller reactivity due to the isotope substitution.

Universal crossed beam imaging studies of polyatomic reaction dynamics.

The marriage between high level quantum calculations and experimental advances in laser technology, quantum state control, and detection techniques have opened the door to the study of molecular collision dynamics at a new level of detail. However, one current challenge lies in adapting these powerful strategies to address questions beyond the scope of the small ground state systems that have largely been the focus of reaction dynamics investigations to-date. For molecules with intermediate or large size (more than 6 atoms), lack of spectroscopic information and spectral congestion limit quantum state preparation, control and detection for experiment, and the large number of degrees of freedom of the system makes accurate quantum dynamics calculations prohibitively expensive. Nevertheless, studies of the chemical dynamics of such systems can reveal novel aspects of reactivity not anticipated based upon the behavior of smaller model systems. This Perspective will highlight applications of soft vacuum ultraviolet photoionization at 157 nm as a universal probe in combination with crossed beams and DC slice velocity map ion imaging to study bimolecular reaction dynamics of molecules of intermediate or large size, illuminated with support of high-level ab initio calculations. Here, we report on the chemical dynamics of atomic oxygen or chlorine reactions with organic compounds: propanol isomers, alkylamines (N(CH3)3 and NH(CH3)2), and isobutene ((CH3)2CCH2) studied using this approach. The polyatomic radical products from the hydrogen abstraction process have been detected by 157 nm photoionization and their slice ion images embody translational energy and angular information that directly reflect the underlying collision dynamics. Various reaction mechanisms (such as direct abstraction and addition-elimination) along with the involvement of roaming dynamics and novel intersystem crossing pathways are presented. These demonstrate the power of this technique to reveal fundamentally new aspects of reaction dynamics that arise in larger reaction systems.

The role of vibronic coupling in the electronic spectroscopy of maleimide: a multi-mode and multi-state quantum dynamics study.

The first two excitation bands below 7 eV in the electronic absorption spectrum of maleimide are investigated using a model Hamiltonian including four low-lying singlet excited states within the manifold of 24 vibrational modes. The role of non-adiabatic effects is studied and shines light on both the broad, inter-state coupling-dominated spectral band as well as the fine-structured, not-so-strong coupled band. Calculations have been performed using the Multiconfigurational Time-Dependent Hartree (MCTDH) wavepacket propagation method as well as its multilayer version (ML-MCTDH) using a quadratic vibronic coupling (QVC) Hamiltonian model where parameters are obtained from fitting adiabatic potential energy surfaces computed by ab initio methods. The quantum dynamics calculations provide information on the relaxation dynamics and the vibrational modes involved. Already with a low-order vibronic coupling model and only a few modes being considered, a quantitative agreement with the experimental spectrum is obtained. However, it is found that all modes need to be considered to get a full picture of the photo-excited relaxation dynamics of this molecule.

Rate coefficients of roaming reaction H+MgH using ring polymer molecular dynamics

The ring-polymer molecular dynamics (RPMD) was used to calculate the thermal rate coefficients of the multi-channel roaming reaction H+MgH→Mg+H2. Two reaction channels, tight and roaming, are explicitly considered. This is a pioneering attempt of exerting RPMD method to multi-channel reactions. With the help of a newly developed optimization-interpolation protocol for preparing the initial structures and adaptive protocol for choosing the force constants, we have successfully obtained the thermal rate coefficients. The results are consistent with those from other theoretical methods, such as variational transition state theory and quantum dynamics. Especially, RPMD results exhibit negative temperature dependence, which is similar to the results from variational transition state theory but different from the ones from ground state quantum dynamics calculations.

Fast optically controlled spin initialization of a quantum dot in the Voigt geometry coupled to a transition metal dichalcogenide monolayer

Abstract We analyze the problem of fast optical spin initialization for a quantum dot in the Voigt geometry by placing it near a transition metal dichalcogenide (TMD) monolayer. We calculate the spontaneous emission rates of the quantum dot modified by the presence of the monolayer for different materials and use the obtained results in quantum dynamics calculations. We show that high levels of fidelity, significantly larger than in the case of the quantum dot in free-space, can be quickly obtained due to the anisotropy of the enhanced spontaneous decay rates of the quantum dot near the TMD monolayer. We use a continuous wave optical field and find the control amplitude which achieves acceptable fidelity levels in short times for different distances of the quantum dot from the TMD monolayer and for various layer materials. We also use state of the art numerical optimal control to find the time-dependent electric field which maximizes the final fidelity for the same short duration and various layer materials as in the previous case. A better fidelity is obtained with this method, while the resulting pulse is quite robust to positioning error of the quantum dot and to additive constant control error.

When Classical Trajectories Get to Quantum Accuracy: The Scattering of H2 on Pd(111).

When elementary reactive processes occur at such low energies that only a few states of reactants and/or products are available, quantum effects strongly manifest and the standard description of the dynamics within the classical framework fails. We show here, for H2 scattering on Pd(111), that by pseudoquantizing in the spirit of Bohr the relevant final actions of the system, along with adequately treating the diffraction-mediated trapping of the incoming wave, classical simulations achieve an unprecedented agreement with state-of-the-art quantum dynamics calculations.

Exact bound rovibrational spectra of the neon tetramer.

Exact quantum dynamics calculations are performed for the bound rovibrational states of the neon tetramer (Ne4) in its ground electronic state, using pair-wise Lennard-Jones potentials and the ScalIT suite of parallel codes. The vibrational states separate into a low-lying group mostly localized to a single potential well and a higher-energy delocalized group lying above the isomerization threshold-with such a structure serving as a testament to the "intermediate" quantum nature of the Ne4 system. To accurately and efficiently represent both groups of states, the phase-space optimized discrete variable representation (PSO-DVR) approach was used, as implemented in the ScalIT code. The resultant 1D PSO effective potentials also shed significant light on the quantum dynamics of the system. All vibrational states were computed well up into the isomerization band and labeled up to the classical isomerization threshold-defined as the addition of the classical energy of a single bond, ϵ = 24.7 cm-1, to the quantum ground state energy. Rovibrational energy levels for all total angular momentum values in the range J = 1-5 were also computed, treating all Coriolis coupling exactly.

Preservation of entanglement and quantum correlations next to periodic plasmonic nanostructures

We study quantum correlations dynamics of two identical V-type quantum systems initially prepared in an extended Werner-like state, where each one independently interacts with a plasmonic nanostructure. Each V-type system can be decomposed as a two-level system with an additional third external level acting as an “umbrella level.” As the plasmonic nanostructure, we use a two-dimensional array of metal-coated dielectric nanoparticles. For the calculations, we combine quantum dynamics calculations using the density matrix equations and classical electromagnetic calculations. In order to describe the entanglement, we use the measure of entanglement of formation, while we use quantum discord to describe the total quantum correlations of our composite system. We find that the presence of the plasmonic nanostructure leads to high suppression of spontaneous emission rates along with a high degree of quantum interference. These phenomena affect the evolution of both entanglement and quantum discord, while they significantly prolong their dynamics.

Double Proton Transfer in the Dimer of Formic Acid: An Efficient Quantum Mechanical Scheme.

Double proton transfer plays an important role in biology and chemistry, such as with DNA base pairs, proteins and molecular clusters, and direct information about these processes can be obtained from tunneling splittings. Carboxylic acid dimers are prototypes for multiple proton transfer, of which the formic acid dimer is the simplest one. Here, we present efficient quantum dynamics calculations of ground-state and fundamental excitation tunneling splittings in the formic acid dimer and its deuterium isotopologues. These are achieved with a multidimensional scheme developed by us, in which the saddle-point normal coordinates are chosen, the basis functions are customized for the proton transfer process, and the preconditioned inexact spectral transform method is used to solve the resultant eigenvalue problem. Our computational results are in excellent agreement with the most recent experiments (Zhang et al., 2017; Li et al., 2019).

Collision-induced absorption in Ar-Kr gas mixtures: A molecular dynamics study with new potential and dipole data.

We have implemented a scheme for classical molecular dynamics simulations of collision-induced absorption. The program has been applied to a gas mixture of argon (Ar) and krypton (Kr). The simulations are compared with accurate quantum dynamical calculations. The comparisons of the absorption coefficients show that classical molecular dynamics is correct within 10% for photon wave numbers up to 220 cm-1 at a temperature of 200 K for this system. At higher temperatures, the agreement is even better. Molecular dynamics accounts for many-body interactions, which, for example, give rise to continuous dimer formation and destruction in the gas. In this way, the method has an advantage compared with bimolecular classical (trajectory) treatments. The calculations are carried out with a new empirical Ar-Kr pair potential. This has been obtained through extensive analysis of experimental thermophysical and transport properties. We also present a new high level ab initio Ar-Kr potential curve for comparison, as well as ab initio interaction-induced dipole curves computed with different methods. In addition, the Ar-Kr polarizability and hyperpolarizability are reported. A comparison of the computed absorption spectra with an experiment taken at 300 K shows satisfactory agreement although a difference in absolute magnitude of 10%-15% persists. This discrepancy we attribute mainly to experimental uncertainty.

Neural-network potential energy surface with small database and high precision: A benchmark of the H + H2 system.

To deeply understand the neural-network (NN) fitting procedure in constructing a potential energy surface (PES) in a wide energy range with a rather small database, based on the existing BKMP2 PES of H + H2, the relationship between NN function features and the size of the database is studied using the multiconfiguration time-dependent Hartree method for quantum dynamics calculations. First, employing 3843, 3843, 2024, and 1448 energy points, four independent NN-PESs are constructed to discuss the relationship among the size of the database, NN functional structure, and fitting accuracy. Dynamics calculations on these different NN PESs give similar reactive probabilities, which indicate that one has to balance the number of energy points for NN training and the number of neurons in the NN function. To explain this problem and try to resolve it, a quantitative model between the data volume and network scale is proposed. Then, this model is discussed and verified through 14 NN PESs fitted using 3843 energy points and various NN functional forms.

Vibronic coupling in the F·CH4 prereactive complex

The F + CH4 → HF + CH3 reaction shows a counter-intuitive mode-selective chemistry and prominent resonances. The prereactive F·CH4 complex formed in the entrance channel is assumed to play an important role in the dynamics of the reaction. The present work investigates the effect of nonadiabatic transitions and the geometric phase on the low-lying quasibound states of the F·CH4 complex. Quantum dynamics calculations employing the multiconfigurational time-dependent Hartree approach and accurately accounting for vibronic as well as spin-orbit coupling are performed. Extending previous work [D. Schäpers and U. Manthe, J. Phys. Chem. A 120, 3186 (2016)], which was restricted to the dynamics on a single adiabatic potential energy surface and found the relative rotation of F and CH4 to proceed almost freely, we found chaotic patterns if vibronic coupling is included. While nonadiabatic transitions strongly affect individual resonances, their effect on averaged quantum state densities and the photodetachment spectrum of F⋅CH4 - is found to be minor.

From Surface Hopping to Quantum Dynamics and Back. Finding Essential Electronic and Nuclear Degrees of Freedom and Optimal Surface Hopping Parameters.

We report an efficient iterative procedure that exploits surface-hopping trajectory methods and quantum dynamics to achieve two complementary purposes: to identify the minimum dimensionality of a molecular Hamiltonian in terms of electronic and nuclear degrees of freedom to study radiationless relaxation mechanisms as well as to provide a reference quantum dynamical calculation that allows assessing of the validity of surface-hopping parameters. This double goal is achieved by a feedback loop between surface hopping and MCTDH calculations based on potential energy surfaces parametrized with a linear vibronic coupling method. Initially, a surface hopping calculation in full dimensionality with a chosen set of parameters is performed, and it is repeated, gradually reducing its dimensionality until divergence with the initial calculation is observed or the system is small enough to be treated quantum dynamically. A comparison between the quantum dynamics and surface hopping simulations dictates the validity of the surface hopping parameters. Using these new parameters, the reduction loop is started again, until convergence. As an example, this strategy is applied to simulate the ultrafast intersystem crossing dynamics of [PtBr6]2- in solution. The 15-dimensional space initially including 200 electronic states is reduced to a 9-dimensional problem with 76 electronic states, without a considerable loss of accuracy.

Non-Markovian spontaneous emission interference near a MoS2 nanodisk.

We propose a nanophotonic structure that gives high-degree quantum interference (QI) in the spontaneous emission (SE) of a quantum emitter (QE) in conjunction with strong light-matter coupling and non-Markovian dynamics. Specifically, we study the SE dynamics of a three-level V-type QE close to a MoS2 nanodisk (ND). We combine quantum dynamics calculations with electromagnetic calculations and find reversible population dynamics in the QE, together with high-degree QI created by the ND. A rich population dynamics is obtained, depending on the energy of the QE with respect to the energies of the exciton-polariton resonances of the MoS2 ND, the distance of the QE from the ND, and the initial state of the QE. Our results have potential applications in emergent quantum technologies.

A computational study of hydrogen dimers in giant-planet infrared spectra

The absorption due to H2–H2 complexes is investigated theoretically. The potential and dipole surfaces for the complex are taken from the literature. Quantum dynamical calculations of the roto-translational absorption spectrum are performed. Special attention is paid to the fine features due to hydrogen dimers, (H2)2, at the centers of the collision-induced rotational S(0) and S(1) transitions. The computed absorption coefficients are used to analyze the spectra of the four giant planets of our solar system.

Quantum dynamics studies of the Na(3S) + HF and Na(3S/3P) + DF reactions: The effects of the initial rotational excitation and the isotopic substitution

Accurate quantum dynamics calculations have been carried out for the Na(3S) + DF(ν = 1–4, j = 0) → NaF + D, Na(3P) + DF(ν = 0, j = 0) → NaF + D/Na(3S) + DF and Na(3S) + HF(ν = 2–4, j = 2,4,6,9) → NaF + H reactions. The isotopic substitution decreases the reactivity remarkably in both Na(3S) + HF and Na(3P) + HF. The calculation also shows that effects of the initial rotational excitations for Na(3S) + HF are different between the process with threshold (v = 2) and those without thresholds (v = 3–4).