Atomic Interaction Potential Research Articles

N-Heterocyclic Carbene-Stabilized Atomically Precise Metal Nanoclusters.

This perspective highlights advances in the preparation and understanding of metal nanoclusters stabilized by organic ligands with a focus on N-heterocyclic carbenes (NHCs). We demonstrate the need for a clear understanding of the relationship between NHC properties and their resulting metal nanocluster structure and properties. We emphasize the importance of balancing nanocluster stability with the introduction of reactive sites for catalytic applications and the importance of a better understanding of how these clusters interact with their environments for effective use in biological applications. The impact of atom-scale simulations, development of atomic interaction potentials suitable for large-scale molecular dynamics simulations, and a deeper understanding of the mechanisms behind synthetic methods and physical properties (e.g., the bright fluorescence displayed by many clusters) are emphasized.

An artificial neural network potential for uranium metal at low pressures

Based on machine learning, the high-dimensional fitting of potential energy surfaces under the framework of first principles provides density-functional accuracy of atomic interaction potential for high-precision and large-scale simulation of alloy materials. In this paper, we obtained the high-dimensional neural network (NN) potential function of uranium metal by training a large amount of first-principles calculated data. The lattice constants of uranium metal with different crystal structures, the elastic constants, and the anisotropy of lattice expansion of alpha-uranium obtained based on this potential function are highly consistent with first-principles calculation or experimental data. In addition, the calculated formation energy of vacancies in alpha- and beta-uranium also matches the first-principles calculation. The calculated site of the most stable self-interstitial and its formation energy is in good agreement with the findings from density functional theory (DFT) calculations. These results show that our potential function can be used for further large-scale molecular dynamics simulation studies of uranium metal at low pressures, and provides the basis for further construction of potential model suitable for a wide range of pressures.

Interaction Potential of Protons and Hydrogen Atoms with Metals

Interaction Potential of Protons and Hydrogen Atoms with Metals

Thermal conductivity across transition metal dichalcogenide bilayers

Cross-plane thermal conductivities of transition metal dichalcogenide (M= Mo, W; X= S, Se) bilayers are determined by homogeneous nonequilibrium molecular dynamics. The best insulator is found to be : , closely followed by : and : . Thermal conductivities of heterobilayers are close to the average of the two corresponding homobilayer values, despite the mass and lattice mismatch. To disentangle the effects of atomic mass, lattice constant, and interaction potential, these three parameters are systematically varied. Phonon spectral analysis provides further insight into their roles and reveals the weak influence of spectral overlap between the two layers and the dominance of boundary scattering. The observed trends can be rationalized using Slack's formula in terms of the average atomic mass and the Debye temperature. Accurate interlayer interaction potentials are developed based on experimental elastic constants. Their effect on the bilayer cross-plane thermal conductivities is found to be minor.

Energy deposition and formation of nanostructures in the interaction of highly charged xenon ions with gold nanolayers

The effect of the deposition of kinetic energy and neutralization energy of slow highly charged xenon ions on the process of the nanostructures creation at the surface of gold nanolayers is investigated. The nanolayers of thickness of 100 nm were prepared by e-beam evaporation of gold on crystalline silicon Si(100) substrate. The samples were irradiated at the Kielce EBIS facility of the Jan Kochanowski University (Kielce, Poland), under high vacuum conditions. The irradiations were performed for constant kinetic energy 280 keV and different ions charge states (Xeq+, q = 25, 30, 35, 36 and 40) and for constant charge state Xe35+ and different kinetic energies: 280 keV, 360 keV, 420 keV and 480 keV. The fluence of the ions was on the level of 1010 ions/cm2. Before and after irradiation the nanolayer surfaces were investigated using the atomic force microscope.As the result, well pronounced modifications of the nanolayer surfaces in the form of craters have been observed. A systematic analysis of the crater sizes (diameter on the surface and depth) allowed us to determine the influence of the deposited kinetic and the neutralization energy on the size of the obtained nanostructures. The results are theoretically interpreted within the micro-staircase model based on the quantum two-state vector model of the ionic Rydberg states population. The charge dependent ion–atom interaction potential inside the solid is used for the calculation of the nuclear stopping power. According to the model the formation of the nanostructures is governed by the processes of the ionic neutralization in front of the surface and the kinetic energy loss inside the solid. The interplay of these two types of processes in the surface structure creation is described by the critical velocity. Using the proposed theoretical model, the neutralization energy, deposited kinetic energy and critical velocities were calculated and compared qualitatively with the experimental results. The results are consistent (after normalization) with previous experimental data and molecular dynamics simulations for single ionized Xe and crystalline gold surface.

Mechanical characterization of nanoporous two-dimensional Ti3C2 MXene membranes

• The Molecular Dynamics with ReaxFF potential were used to investigate the mechanical properties of nanoporous two-dimensional Ti 3 C 2 MXene. • Young's model decreases with the increase of the area of the nanoporous. • The fracture pressure decrease with the increase of separation distance in the zigzag direction due to anisotropy in the structure of MXene. • The nanoporous Ti 3 C 2 MXene membrane can maintain its mechanical integrity in the reverse osmosis desalination process. We performed a molecular dynamics simulation to study the mechanical properties of nanoporous two-dimensional Ti 3 C 2 MXene membranes. The aim is to find out whether these membranes are strong enough to maintain their mechanical integrity in the reverse osmosis desalination process. We modeled the atomic interaction potential of the Ti 3 C 2 system using a reactive force field (ReaxFF) potential. The obtained results indicate that the ultimate strength of nanoporous MXene membranes decreases with an increase in the nanoporous surface and the separation distance in the armchair direction. However, it increases with the decrease of separation distance following the zigzag direction. The Young's modulus of Ti 3 C 2 with a grating nanoporous (112 rectangle pores) was found to be equal to 475.699 GPa and 421.008 GPa, and the calculated fracture pressures at 300K were equal to 30.09 GPa and 28.25 GPa for porous with 28.666 Å 2 and 38.639 Å 2 respectively. The simulation findings are in good agreement with experimental ones for a sample without pores.

Dynamics of atoms within atoms

Recent experiments with Bose–Einstein condensates have entered a regime in which thousands of ground-state condensate atoms fill the Rydberg-electron orbit. After the excitation of a single atom into a highly excited Rydberg state, scattering off the Rydberg electron sets ground-state atoms into motion, such that one can study the quantum-many-body dynamics of atoms moving within the Rydberg atom. Here we study this many-body dynamics using Gross–Pitaevskii and truncated Wigner theory. Our simulations focus in particular on the scenario of multiple sequential Rydberg excitations on the same rubidium condensate which has become the standard tool to observe quantum impurity dynamics in Rydberg experiments. We investigate to what extent such experiments can be sensitive to details in the electron–atom interaction potential, such as the rapid radial modulation of the Rydberg molecular potential, or p-wave shape resonance. We demonstrate that both effects are crucial for the initial condensate response within the Rydberg orbit, but become less relevant for the density waves emerging outside the Rydberg excitation region at later times. Finally we explore the local dynamics of condensate heating. We find that it provides only minor corrections to the mean-field dynamics. Combining all these insights, our results suggest Bose–Einstein condensates as a viable platform for the in situ and real time interrogation of ultra-cold chemistry dynamics involving Rydberg states.

Static investigation of the small clusters on the Cu(111) and Au(111) surfaces

The diffusion properties of small clusters Agn, Cun, and Aun on the Cu(111) and Au(111) surfaces were studied using the molecular statics (MS) in order to understand the atomistic processes underlying the motion. In this work, the atomic interaction potential is modeled by a semi-empirical Embedded Atom Method (EAM), while the drag method is used to determine the static activation energy for each diffusion path. The presented results indicate that the dimer can diffuse on the (111) surface via the zig-zag and concerted motion mechanisms. The trimer diffuses according to the concerted motion mechanism. For the tetramer diffusion, the mechanism that consumes the least amount of energy is the zig-zag motion, in which only two atoms are needed for the diffusion process at one time. This allows finding a static activation energy smaller compared to the trimer diffusion.

Assessment of the classical nucleation theory in supercooled nickel by molecular dynamics

In this work, we performed a thorough study of crystal nucleation rates in supercooled liquid (SCL) nickel by comparing molecular dynamics (MD) simulation results, theoretical calculations, and experimental data. Our aim was to assess the validity of the most used theoretical framework, the Classical Nucleation Theory (CNT), in describing homogeneous nucleation in a deeply supercooled liquid. To this end, we carried out MD simulations using an embedded atom interaction potential, which describes quite well the melting point, Tm, the enthalpy of fusion, Δh, and the liquid density, ρl, of our model material. We then determined the critical nucleus size and the diffusion coefficient in the temperature range from 0.68 to 0.80Tm. By combining the physical properties obtained from our MD simulations and the calculated crystal-liquid free energy difference, we compared the CNT predictions with the nucleation rates obtained directly via brute force MD simulations on a system with 4 million atoms. We found an excellent agreement between the theoretically calculated nucleation rates and the values obtained by MD simulations. Moreover, the CNT prediction for the nucleation rate agrees with experimental data at 1350 K. Our results confirm that the CNT is indeed a powerful tool to describe the nucleation kinetics in supercooled liquids.

Effect of surface temperature on quantum dynamics of H2 on Cu(111) using a chemically accurate potential energy surface.

The effect of surface atom vibrations on H2 scattering from a Cu(111) surface at different temperatures is being investigated for hydrogen molecules in their rovibrational ground state (v = 0, j = 0). We assume weakly correlated interactions between molecular degrees of freedom and surface modes through a Hartree product type wavefunction. While constructing the six-dimensional effective Hamiltonian, we employ (a) a chemically accurate potential energy surface according to the static corrugation model [M. Wijzenbroek and M. F. Somers, J. Chem. Phys. 137, 054703 (2012)]; (b) normal mode frequencies and displacement vectors calculated with different surface atom interaction potentials within a cluster approximation; and (c) initial state distributions for the vibrational modes according to Bose-Einstein probability factors. We carry out 6D quantum dynamics with the so-constructed effective Hamiltonian and analyze sticking and state-to-state scattering probabilities. The surface atom vibrations affect the chemisorption dynamics. The results show physically meaningful trends for both reaction and scattering probabilities compared to experimental and other theoretical results.

VPA: Computer program for the computation of the phase shift in atom–atom potential scattering using the Variable Phase Approach

A computer code for the computation of the phase shift in atom–atom and electron–atom potential scattering is presented. The phase shift is the central quantity required for the calculation of all types of scattering cross sections. The program uses the Variable Phase Approach (VPA). This is the only exact method for the direct calculation of the scattering phase shift. All other methods are based on examining the large distance behavior of the exact solution of the Schrödinger equation. Such methods yield the phase shift only modulo π. The absolute value of the phase shift and its variation with scattering energy is, however, needed for a full understanding of the scattering process, such as for instance in the study of shape resonances and Glory oscillations. The VPA has been sparingly used owing to the instability of the underlying equations and the consequent difficulty of writing computer code to solve them. We present a computer code for the efficient implementation of the VPA method for atom–atom scattering problems over a wide range of scattering energies. The code works for potentials which are singular and for those that are non-singular at the origin. An example of the implementation of the code is given for both an interaction potential with an attractive well and for a purely repulsive potential. Program summaryProgram title: VPA:Variable Phase ApproachCPC Library link to program files: https://dx.doi.org/10.17632/zh248d2362.1Code Ocean capsule: https://dx.doi.org/10.24433/CO.8146850.v1Licensing provisions: GNU General Public License 3 (GPL)Programming language: Fortran 90Nature of problem: To compute the quantum mechanical phase shift in an atom–atom or an electron–atom potential scattering problem. The interaction potential, dependent on a single radial coordinate, is defined by the user.Solution method: The Variable Phase Approach is used. This is the only available method for the direct calculation of the phase shift. The initial internuclear separation for the integration is chosen with the help of the analytical solution for a hard sphere core for all orbital angular momentum quantum numbers L of interest. The first-order nonlinear equation is integrated numerically with a modified LSODA19,20 program. The analytic formula for calculation of the Jacobian of the equation is also provided.Additional comments including restrictions and unusual features: Computer codes for the atom–atom interaction potential, POTENT1, and for its derivative with respect to the inter nuclear separation, DPOTENT1, must be supplied by a user. The calculation of the spherical Bessel and Neumann functions required is performed explicitly for large l angular momentum quantum numbers where the standard upward iterative generation of the functions becomes unstable.

ARC 3.0: An expanded Python toolbox for atomic physics calculations

ARC 3.0 is a modular, object-oriented Python library combining data and algorithms to enable the calculation of a range of properties of alkali and divalent atoms. Building on the initial version of the ARC library (Šibalić et al., 2017), which focused on Rydberg states of alkali atoms, this major upgrade introduces support for divalent atoms. It also adds new methods for working with atom–surface interactions, for modelling ultracold atoms in optical lattices and for calculating valence electron wave functions and dynamic polarisabilities. Such calculations have applications in a variety of fields, e.g., in the quantum simulation of many-body physics, in atom-based sensing of DC and AC fields (including in microwave and THz metrology) and in the development of quantum gate protocols. ARC 3.0 comes with an extensive documentation including numerous examples. Its modular structure facilitates its application to a wide range of problems in atom-based quantum technologies. Program summaryProgram Title: ARC 3.0CPC Library link to program files:https://doi.org/10.17632/c4z4n2cdf7.1Licencing provisions: BSD-3-ClauseProgramming language: PythonExternal Routines: NumPy [1], SciPy [1], Matplotlib [2], SymPy [3], LmFit [4]Nature of problem: The calculation of atomic properties of alkali and divalent atoms including energies, Stark shifts and dipole–dipole interaction strengths using matrix elements evaluated through a variety of means.Solution method: Dipole matrix elements are calculated using an analytical semi-classical approximation or wave functions obtained by numerical integration of the radial Schrödinger equation for a one-electron model potential. Interaction energies and shifts due to external fields are calculated using second order degenerate perturbation theory or exact diagonalisation of the interaction Hamiltonian, yielding results valid even at large external fields or small interatomic separation.Additional comments including restrictions and unusual features: External electric and magnetic field must be parallel to the quantisation axis. The accuracy of short range (≲1μm) atom - atom interaction potentials is limited by the truncation of the basis. Only weak magnetic fields are supported as only linear Zeeman shifts are taken into account. Calculations for divalent atoms use a single-electron approximation and calculation of their wave functions is not supported.

Scattering of Fast Atoms and Ions by a Crystalline Surface: Orientation Effects

The features of the scattering of accelerated neutral atoms upon sliding interactions with crystalline surfaces are discussed. Particular attention is paid to the phenomenon of rainbow scattering of neutral atoms upon their reflection from an oriented crystalline surface. The physical causes of the arising effects and the possibility of reconstructing the two-particle “polarization” and “dynamic” ion–atom interaction potentials from the experimental data are considered.

Heat transport in carbon nanotubes: Length dependence of phononic conductivity from the Boltzmann transport equation and molecular dynamics

In this article, we address lattice heat transport in single-walled carbon nanotubes (CNTs) by a quantum mechanical calculation of three-phonon scattering rates in the framework of the Boltzmann transport equation (BTE) and classical molecular dynamics (MD) simulation. Under a consistent choice of an empirical, realistic atomic interaction potential, we compare the tube length dependence of the lattice thermal conductivity (TC) at room temperature determined from an iterative solution of the BTE and from a nonequilibrium MD (NEMD) approach. Qualitatively similar trends are found in the limit of short tubes, where an extensive regime of ballistic heat transport prevailing in CNTs of lengths $L\lesssim 1\,\rm{\mu m}$ is independently confirmed. In the limit of long tubes, the BTE approach suggests a saturation of TC with tube length, whereas direct NEMD simulations of tubes extending up to $L=10\,\rm{\mu m}$ are demonstrated to be insufficient to settle the question of whether a fully diffusive heat transport regime and an intrinsic value of TC exist for CNTs. Noting that acoustic phonon lifetimes lie at the heart of a saturation of TC with tube length as per the BTE framework, we complement the quantum mechanical prediction of acoustic phonon lifetimes with an analysis of phonon modes in the framework of equilibrium MD (EMD). A normal mode analysis (NMA) with an emphasis on long wavelength acoustic modes corroborates the BTE prediction that heat transport in CNTs in the long tube limit is governed by the low attenuation rates of longitudinal and twisting phonons.

H–He collision-induced satellite in the Lyman α profile of DBA white dwarf stars

ABSTRACT The spectra of helium-dominated white dwarf stars with hydrogen in their atmosphere present a distinctive broad feature centred around 1160 Å in the blue wing of the Lyman α line. It is extremely apparent in WD 1425+540 recently observed with Hubble Space Telescope(HST) Cosmic Origins Spectrograph (COS). With new theoretical line profiles based on ab initio atomic interaction potentials we show that this feature is a signature of a collision-induced satellite due to an asymptotically forbidden transition. This quasi-molecular spectral satellite is crucial to understanding the asymmetrical shape of Lyman α seen in this and other white dwarf spectra. Our previous work predicting this absorption feature was limited by molecular potentials that were not adequate to follow the atomic interactions with spectroscopic precision to the asymptotic limit of large separation. A new set of potential energy curves and electronic dipole transition moments for the lowest electronic states of the H–He system were developed to account accurately for the behaviour of the atomic interactions at all distances, from the chemical regime within 1 Å out to where the radiating H atoms are not significantly perturbed by their neighbours. We use a general unified theory of collision-broadened atomic spectral lines to describe a rigorous treatment of hydrogen Lyman α with these potentials and present a new study of its broadening by radiative collisions of hydrogen and neutral helium. These results enable ab initio modelling of radiative transport in DBA white dwarf atmospheres.

The directional motion of nano-objects induced by an inhomogeneous strain field

Recent molecular dynamics simulations have shown that an inhomogeneous strain field in a substrate could drive nano-objects to move on its surface. A theoretical model is established in this paper in order to find the mechanical mechanism from the continuum mechanics point of view, but with the help of atomic interaction potential. The substrate is represented by graphene, and the above nano-object is represented by a graphene flake or a carbon nanotube. Three typically inhomogeneous strain fields are considered, including linear, power and exponential. It was found that a driving force and a couple moment can be induced on the flat flake and the circular nanotube, respectively. Furthermore, both the driving force and the couple moment increase with the increase in strain gradient in the substrate. The size effect of nano-objects and boundary effect on the movement are also investigated in order to explain some interesting results found in the existing numerical simulations. The present theoretical study cannot only reveal the source of the driving mechanism for the movement induced by an inhomogeneous strain field but also give insights for the design of nano-transport devices.

Cross-sections, transport coefficients and dissociation rate constants for molecular ion interacting with Kr

A hybrid method based on quantum mechanical formalism for electrons and a classical treatment for nuclei was used to study the dynamics of electronically ground state krypton dimer in collision with its parent gas. Two sets of collision cross-sections, namely non-reactive scattering and collision induced dissociation, were calculated from this method by using different sets of atom–atom interaction potentials (neutral–neutral, ion–neutral). These cross-sections were injected in Monte Carlo simulations for the determination of mobility, diffusion and dissociation rate constant. It is noteworthy that we also used an inverse method based on JWKB approximation to calculate a global momentum transfer collision cross-section without discerning the inelastic processes as dissociation. Thus, the transport coefficients obtained are also in a pretty good agreement with experimental data. All Monte Carlo simulations were performed at ambient pressure and temperature over a broad range of reduced electric field. Moreover, effects of rovibronic excitations were also studied thoroughly in order to improve correlation between theoretical mobility calculations and experimental data available in the literature.

Dynamic Potential of the Interaction between Neutral Atoms and the Surface of a Crystal

The parameters of the dynamic ion–atomic interaction potential are obtained in the analytical form for a N–Al system in the energy range of 10–35 keV, using the rainbow scattering angle as a function of the energy of particles incident on a (111) aluminum crystal surface in the [1 $$\overline 1 $$ 0] direction. The calculation results are found to coincide with available experimental data.

Library-Based LAMMPS Implementation of High-Dimensional Neural Network Potentials.

Neural networks and other machine learning approaches have been successfully used to accurately represent atomic interaction potentials derived from computationally demanding electronic structure calculations. Due to their low computational cost, such representations open the possibility for large scale reactive molecular dynamics simulations of processes with bonding situations that cannot be described accurately with traditional empirical force fields. Here, we present a library of functions developed for the implementation of neural network potentials. Written in C++, this library incorporates several strategies resulting in a very high efficiency of neural network potential-energy and force evaluations. Based on this library, we have developed an implementation of the neural network potential within the molecular dynamics package LAMMPS and demonstrate its performance using liquid water as a test system.

Вероятность процессов захвата электронов у атомов аргона ионами -=SUP=-3-=/SUP=-Не-=SUP=-2+-=/SUP=- при различных параметрах удара

AbstractWe have measured the absolute values of total cross sections of capture of one and two electrons by He^2+ ions from argon atoms. The differential scattering cross sections have been determined for fast atoms and singly charged helium ions formed in each of these processes (without and with additional ionization of the formed slow argon ion). Measurements have been taken for He^2+ ions with kinetic energy of 6 keV in scattering angle range 0–2.5°. Based on the measured differential cross sections using different model atomic particle interaction potentials, we have calculated the cross sections of these processes as functions of the impact parameter. The probabilities of realization of these processes with electron density distribution in different shells in the target atom have been compared. The applicability of the expressions for the screened Coulomb interaction potentials in the description of scattering of particles that have captured electrons has been demonstrated.