Quantum Mechanical Electron Research Articles

Playing Catch with Electrons: A Dynamic Metaphor for Chemical Bonding with an Extension of the Tunneling Model to Gas-Phase Molecules.

Quantum mechanical electron tunneling is explored as the mediator of chemical bonding. Three topics are addressed. First, a baseball game-of-catch metaphor is employed to elucidate the physics of chemical bonding. According to this metaphor, a baseball player, a baseball, and the act of throwing a baseball are analogous to an atom, an electron, and the act of electron tunneling. Using this framework, the tunneling physics of covalent, polar covalent, ionic, and hydrogen bonding is clarified. Second, gas-state energies associated with frontier orbital positioning are established and are used in the chemical bond tunneling assessment of four homopolar bonds, H-H, N≡N, O=O, and F-F, and four heteropolar bonds, H-F, H-Cl, H-Br, and H-I. Third, a new procedure for estimating solid-state tunneling parameters is introduced, involving quantum confinement of gas-state energy frontier orbitals.

Exchange–correlation entropy from the generalized thermal adiabatic connection

Warm dense matter is a highly energetic phase characterized by strong correlations, thermal effects, and quantum mechanical electrons. Thermal density functional theory is commonly used in simulations of this challenging phase, driving the development of temperature-dependent approximations to the exchange–correlation free energy. Approaches using the adiabatic connection formula are well known at zero temperature and have been recently leveraged at non-zero temperatures as well. In this work, a generalized thermal adiabatic connection (GTAC) formula is proposed, introducing a fictitious temperature parameter. This allows extraction of the exchange–correlation entropy SXC using simulated interaction strength scaling. This procedure uses a Hellmann–Feynman approach to express the exchange–correlation entropy in terms of a temperature- and interaction strength-dependent exchange–correlation potential energy. In addition, analysis of SXC as a function of interaction strength suggests new forms for approximations, and GTAC itself offers a new framework for exploring both the exact and approximate interplay of temperature, density, and interaction strength across a wide range of conditions.

Quantum mechanical treatment for potential antiphlogistic effects from the leaf extract of Ocimum basilicum citriodorum using gas chromatography-mass spectrometry (GCMS)

Abstract The immune biological response systems and inflammation can be triggered by a number of things such as pathogens, damaged cells and toxic substances. In ethnomedicine, leaves of Lemon basil (Ocimum basilicum citriodorum) have been used for their health benefits. This study examines the anti-inflammatory properties of lemon basil stalks. In a GC/MS study, 24 plant-based bioactive compounds were identified. Comparing the activities in 24 compounds with the largest negative binding energy values helped us determine which compound was most active. It can be seen that only two compounds (Campstool and stigmasterol) with the highest binding energies interact with the 2QVD protein; consequently, the compound with the highest binding energy has superior anti-inflammatory activity. In quantum mechanics, electron energy difference between lowest-unoccupied molecular orbitals (LUMO) and highest-occupied molecular orbitals (HOMO) is described by the quantum-mechanical method, electronegativity (χ), electron affinity (A), global hardness (η), global softness (σ) and ionization potential (I) estimated from the optimized structure. Higher energy molecules are more reactive than other substances reported in this study.

A Density Potential Functional Approach for Mesoscopic Metal-Solution Interfaces

Mesoscopic metal-solution interfaces (MSI) are found at nanostructured electrodes in myriad electrochemical devices. Resolving the local reaction environment for electrochemical reactions occurring in mesoscopic MSIs requires a constant-potential, computationally economic method that properly treat both quantum mechanical metal electrons and almost classical electrolyte components. We have been developing a density potential functional theory (DPFT) to meet these harsh requirements [1-4]. Our DPFT method combines an orbital-free density functional theory of quantum mechanical metal electrons, a statistical field theoretic description of classical electrolyte ions and solvent, and Morse potentials describing short-range interactions between the two parts that are parameterized using first-principles based computations. In this talk, I will introduce the theoretical framework, its relationship to other similar methods including the jellium model and implicit solvation model, its parameterization and benchmark with experimental and ab initio simulation data, and recent applications to understand solvent effects on potential of zero charge, mechanoelectrochemistry of MSIs, and supported nanoparticle catalysts. The emerging opportunities, future development needs, and important caveats of the DPFT method will be discussed. Reference Huang J, Zhang Y, Li M, Groß A, Sakong S. Comparing Ab Initio Molecular Dynamics and a Semiclassical Grand Canonical Scheme for the Electric Double Layer of the Pt (111)/Water Interface. The Journal of Physical Chemistry Letters. 2023 Feb 27;14:2354-63.Huang J. Density-Potential Functional Theory of Electrochemical Double Layers: Calibration on the Ag (111)-KPF6 System and Parametric Analysis. Journal of Chemical Theory and Computation. 2023 Jan 18.Huang J, Chen S, Eikerling M. Grand-canonical model of electrochemical double layers from a hybrid density–potential functional. Journal of chemical theory and computation. 2021 Mar 31;17(4):2417-30.Huang J. Hybrid density-potential functional theory of electric double layers. Electrochimica Acta. 2021 Sep 1;389:138720. J.H. is supported by the Helmholtz Young Investigators Group Leader Program.

Bonding and Tunneling.

Quantum mechanical electron tunneling is proposed as the mediator of chemical bonding. Covalent, ionic, and polar covalent bonds all rely on quantum mechanical tunneling, but the nature of tunneling differs for each bond type. Covalent bonding involves bidirectional tunneling across a symmetric energy barrier. Ionic bonding occurs by unidirectional tunneling from the cation to the anion across an asymmetric energy barrier. Polar covalent bonding is a more complicated type of bidirectional tunneling, consisting of both cation-to-anion and anion-to-cation tunneling across asymmetric energy barriers. Tunneling considerations suggest the possibility of another type of bond-denoted polar ionic-in which tunneling involves two electrons across asymmetric barriers.

Optical response of metallic nanostructures using quantum hydrodynamic theory and a hybridizable discontinuous Galerkin method

An accurate modeling of the optical interactions in metallic nanostructures with subnanometer features requires an accurate description of quantum effects at the scale of billions of atoms. At such scale, first-principle methods are not computationally viable. Quantum hydrodynamic theory (QHT) has emerged as a powerful method that includes nonlocal contributions of the kinetic energy and the spatial dependence of the electron density, and it can predict both plasmon energy and spill-out effects in large metal nanoparticles. In this paper, we introduce a hybridizable discontinuous Galerkin method for solving Maxwell's equations coupled with a QHT model in order to account for quantum effects in three-dimensional metallic nanostructures. The coupled system of Maxwell's equations and QHT model is not only nonlinear but also multi-scale due to the interaction between the micrometer electromagnetic waves and the nanometer cavities of metallic nanostructures. We present extensive numerical experiments to validate the QHT model and demonstrate the capability of the HDG method to provide accurate solutions in the presence of strong nonlinearities and multiple length scales. These results offer a possibility to enhance nonlinear optical effects or to harness quantum mechanical electron tunneling by engineering metallic nanostructures at the quantum level.

Charge trapping effect at the interface of ferroelectric/interlayer in the ferroelectric field effect transistor gate stack

We study the charge trapping phenomenon that restricts the endurance of n-type ferroelectric field-effect transistors (FeFETs) with metal/ferroelectric/interlayer/Si (MFIS) gate stack structure. In order to explore the physical mechanism of the endurance failure caused by the charge trapping effect, we first establish a model to simulate the electron trapping behavior in n-type Si FeFET. The model is based on the quantum mechanical electron tunneling theory. And then, we use the pulsed I d–V g method to measure the threshold voltage shift between the rising edges and falling edges of the FeFET. Our model fits the experimental data well. By fitting the model with the experimental data, we get the following conclusions. (i) During the positive operation pulse, electrons in the Si substrate are mainly trapped at the interface between the ferroelectric (FE) layer and interlayer (IL) of the FeFET gate stack by inelastic trap-assisted tunneling. (ii) Based on our model, we can get the number of electrons trapped into the gate stack during the positive operation pulse. (iii) The model can be used to evaluate trap parameters, which will help us to further understand the fatigue mechanism of FeFET.

Angstrom-scale imaging of magnetization in antiferromagnetic Fe2As via 4D-STEM

We demonstrate a combination of computational tools and experimental 4D-STEM methods to image the local magnetic moment in antiferromagnetic Fe2As with 6 angstrom spatial resolution. Our techniques utilize magnetic diffraction peaks, common in antiferromagnetic materials, to create imaging modes that directly visualize the magnetic lattice. Using this approach, we show that center-of-mass analysis can determine the local magnetization component in the plane perpendicular to the path of the electron beam. Moreover, we develop Magnstem, a quantum mechanical electron scattering simulation code, to model electron scattering of an angstrom-scale probe from magnetic materials. Using these tools, we identify optimal experimental conditions for separating weak magnetic signals from the much stronger interactions of an angstrom-scale probe with electrostatic potentials. Our techniques should be useful for characterizing the local magnetic order in systems such in thin films, interfaces, and domain boundaries of antiferromagnetic materials, which are difficult to probe with existing methods.

Geometry of Charge Density as a Reporter on the Role of the Protein Scaffold in Enzymatic Catalysis: Electrostatic Preorganization and Beyond.

Enzymes host active sites inside protein macromolecules, which have diverse, often incredibly complex, and atom-expensive structures. It is an outstanding question what the role of these expensive scaffolds might be in enzymatic catalysis. Answering this question is essential to both enzymology and the design of artificial enzymes with proficiencies that will match those of the best natural enzymes. Protein rigidifying the active site, contrasted with the dynamics and vibrational motion promoting the reaction, as well as long-range electrostatics (also known as electrostatic preorganization) were all proposed as central contributions of the scaffold to the catalysis. Here, we show that all these effects inevitably produce changes in the quantum mechanical electron density in the active site, which in turn defines the reactivity. The phenomena are therefore fundamentally inseparable. The geometry of the electron density-a scalar field characterized by a number of mathematical features such as critical points-is a rigorous and convenient descriptor of enzymatic catalysis and a reporter on the role of the protein. We show how this geometry can be analyzed, linked to the reaction barriers, and report in particular on intramolecular electric fields in enzymes. We illustrate these tools on the studies of electrostatic preorganization in several representative enzyme classes, both natural and artificial. We highlight the forward-looking aspects of the approach.

Influence of modelling disorder on Hirshfeld atom refinement results of an organo-gold(I) compound.

Details of the validation of disorder modelling with Hirshfeld atom refinement (HAR) for a previously investigated organo-gold(I) compound are presented here. The impact of refining disorder on HAR results is discussed using an analysis of the differences of dynamic structure factors. These dynamic structure factor differences are calculated from thermally smeared quantum mechanical electron densities based on wavefunctions that include or exclude electron correlation and relativistic effects. When disorder is modelled, the electron densities stem from a weighted superposition of two (or more) different conformers. Here this is shown to impact the relative importance of electron correlation and relativistic effect estimates expressed by the structure factor magnitudes. The role of disorder modelling is also compared with the effect of the treatment of hydrogen anisotropic displacement parameter (ADP) values and atomic anharmonicity of the gold atom. The analysis of ADP values of gold and disordered carbon atoms showed that the effect of disorder significantly altered carbon ADP values and did not influence those of the gold atom.

Chemistry without the Born-Oppenheimer approximation.

Chemistry without the Born-Oppenheimer approximation.

Wavefunction-Based Electrostatic-Embedding QM/MM Using CFOUR through MiMiC.

We present an interface of the wavefunction-based quantum chemical software CFOUR to the multiscale modeling framework MiMiC. Electrostatic embedding of the quantum mechanical (QM) part is achieved by analytic evaluation of one-electron integrals in CFOUR, while the rest of the QM/molecular mechanical (MM) operations are treated according to the previous MiMiC-based QM/MM implementation. Long-range electrostatic interactions are treated by a multipole expansion of the potential from the QM electron density to reduce the computational cost without loss of accuracy. Testing on model water/water systems, we verified that the CFOUR interface to MiMiC is robust, guaranteeing fast convergence of the self-consistent field cycles and optimal conservation of the energy during the integration of the equations of motion. Finally, we verified that the CFOUR interface to MiMiC is compatible with the use of a QM/QM multiple time-step algorithm, which effectively reduces the cost of ab initio MD (AIMD) or QM/MM-MD simulations using higher level wavefunction-based approaches compared to cheaper density functional theory-based ones. The new wavefunction-based AIMD and QM/MM-MD implementations were tested and validated for a large number of wavefunction approaches, including Hartree-Fock and post-Hartree-Fock methods like Møller-Plesset, coupled-cluster, and complete active space self-consistent field.

H-Atom Assignment and Sb-O Bonding of [Mes3SbOH][O3SPh] Confirmed by Neutron Diffraction, Multipole Modeling, and Hirshfeld Atom Refinement.

Neutron wavelength-resolved Laue diffraction experiments permit accurate refinement of the H-atom positions and anisotropic displacement parameters of [Mes3SbOH][O3SPh]. A multipole-based charge density refinement and a topological analysis of the refined electron density were also performed. Hirshfeld atom refinement (HAR) recovers the neutron-determined H-atom parameters, and the quantum-mechanical electron density used in HAR recovers the electron density topology from the refined multipole model. These results confirm that [Mes3SbOH][O3SPh] does indeed feature a hydroxystibonium cation with a nominal Sb-O single bond and not a stibine oxide with an Sb=O/Sb+-O- bond.

The advanced treatment of hydrogen bonding in quantum crystallography.

Although hydrogen bonding is one of the most important motifs in chemistry and biology, H-atom parameters are especially problematic to refine against X-ray diffraction data. New developments in quantum crystallography offer a remedy. This article reports how hydrogen bonds are treated in three different quantum-crystallographic methods: Hirshfeld atom refinement (HAR), HAR coupled to extremely localized molecular orbitals and X-ray wavefunction refinement. Three different compound classes that form strong intra- or intermolecular hydrogen bonds are used as test cases: hydrogen maleates, the tripeptide l-alanyl-glycyl-l-alanine co-crystallized with water, and xylitol. The differences in the quantum-mechanical electron densities underlying all the used methods are analysed, as well as how these differences impact on the refinement results.

Heterogeneous Intramolecular Electric Field as a Descriptor of Diels-Alder Reactivity.

External electric fields have proven to be an effective tool in catalysis, on par with pressure and temperature, affecting the thermodynamics and kinetics of a reaction. However, fields in molecules are complicated heterogeneous vector objects, and there is no universal recipe for grasping the exact features of these fields that implicate reactivity. Herein, we demonstrate that topological features of the heterogeneous electric field within the reactant state and of the quantum mechanical electron density-a scalar reporter on the field experienced by the system-can be identified as rigorous descriptors of the reactivity to follow. We scrutinize specifically the Diels-Alder reaction. Its 3D nature and the lack of a singular directionality of charge movement upon barrier crossing make the effect of the electric field not obvious. We show that the electric field topology around the dienophile double bond and the associated changes in the topology of the electron density in this bond are predictors of the reaction barrier. They are also the metrics to rationalize and predict how the external field would inhibit or enhance the reaction. The findings pave the way toward designing external fields for catalysis and reading the reactivity without an explicit mechanistic interrogation, for a variety of reactions.

Out-of-plane corrugations in graphene based van der Waals heterostructures

Graphene, the first truly two-dimensional material isolated, is never perfectly flat. Even when it is sandwiched between other atomically flat crystals, it still slightly ripples. These out-of-plane corrugations, deformations of the graphene layer into the third dimension, have profound implications on graphene's properties. They reduce the mobility of electrons moving within graphene just like bumps on a road slow down a car. Here, the authors demonstrate the presence of such corrugations by studying the quantum mechanical phase electrons acquire in the presence of a magnetic field.

A novel method for analysis of the electrostatic complementarity of protein-protein interaction based on fragment molecular orbital method

Here, the development of a method for analyzing the electrostatic complementarity of protein-protein interactions (PPIs) using fully quantum mechanical electron density (EDN) and electrostatic potential (ESP) is described. Partial EDN (pEDN) and ESP (pESP) of each protein were newly defined based on the fragment molecular orbital method. Illustrative calculations were performed for programmed cell death-1 (PD-1) and its ligand (PD-L1), which showed that the electrostatic complementarity of PD-1 and PD-L1 was clearly represented by the pEDN and pESP. Further analysis revealed that electrostatic complementarity induced by complex formation is important for binding between the proteins.

Wannier-Stark ladder spectrum of Bloch oscillations of magneto-dipole spin waves in graded 1D magnonic crystals

We have used the method of Wannier functions to calculate the frequencies and profiles of spin waves localized in one-dimensional magnonic crystals due to a gradient in the bias magnetic field. This localization of spin waves is analogous to the phenomenon of Bloch oscillations of quantum-mechanical electrons in crystals in a uniform electric field. As a convenient yet realistic model, we consider backward volume magnetostatic spin waves in a film of yttrium-iron garnet in a bias magnetic field comprising spatially uniform, cosine and gradient contributions. The spin-wave spectrum is shown to have the characteristic form of a Wannier–Stark ladder. The analytical results are verified using those obtained using numerical micromagnetic simulations. The physics of spin-wave Bloch oscillations combines the topics of magnonic crystals and graded magnonic index — the two cornerstones of modern magnonics.

The Nanomechanical Bit.

The applicability of nanomechanical devices for computational approaches is reviewed. The focus is on the representation and processing of information based on nanomechanical bits. Several device concepts are discussed ranging from nano-electromechanical systems in silicon to circuits based on carbon nano-tube switches, combinations of nanomechanical resonators and traditional transistors, and integration into a computing architecture. The strengths of mechanical systems include their scalability, robustness to external electrical shocks, and their low-energy consumption. Hence, they may lead the way to new forms of ultradense memory and alternative routes of computing. In conjunction with quantum mechanical single electron circuits, nano-electromechanical systems may also have potential for quantum computational circuits.

Dephasing due to semi-conductor charging masks decoherence in electron-wall systems

In the interaction between a quantum mechanical electron wave and a wall, loss of interference contrast is observed. It is of general interest to identify the mechanism and categorize its nature as dephasing or decoherence. Decoherence is of fundamental importance for the transition between the classical and quantum boundary, while dephasing is often an experimental detriment that needs to be understood and overcome. We find that the loss of contrast is attributed to a local charge distribution induced by secondary emission: a dephasing mechanism. This mechanism is expected to be present for many materials, is clearly visible for copper oxide, and can mask decoherence mechanisms. The local charge distribution on the wall causes a loss of contrast and electron beam deformation. A model based on Coulomb interaction with the surface charge gives good agreement for both features. Our model can thus be used to improve the experimental design and lead to separate out decoherence phenomena in the electron-wall system.