Dynamics Simulations Of Collisions Research Articles

A possible new molecular mechanism of thundercloud electrification

Thunderclouds are electrified when charge is transferred between small and large ice particles colliding in a cloud that contains strong updrafts. The small ice particles rise with one type of charge and the large ice particles fall and carry with them downward the other type of charge, which is most often negative, so that normally lightning lowers negative charge from cloud to the ground. While the collisional mechanism of thundercloud charging is well established, the nature of the charge transfer between the colliding ice particles is not very well understood on the atomic level, and no present theory can explain it in full detail. Here we propose a new charge separation mechanism that is based on molecular dynamics simulations of particle surfaces and collisions, keeping track of the individual charges as they move in the form of salt ions from one ice particle to another. Under normal conditions, when sulfates dominate as cloud condensation nuclei, this ionic mechanism is consistent with the prevailing negative charging of graupels in thunderclouds. Moreover, with dearth of sulfate anions, the present mechanism predicts a shift towards positive charging. This fits well to a large range of observations of enhanced positive lightning, connected with smoke rich in chlorides and nitrates, that could not be explained satisfactorily previously.

Kalypso: a software package for molecular dynamics simulation of atomic collisions at surfaces

A suite of graphically-oriented computer programs (Kalypso) has been developed for molecular dynamics simulations of projectile collisions with metallic targets. The features of the package include free-boundary or periodic targets, many-body potentials, a variety of inelastic loss models, and the ability to carry out simulations of fluence-dependent processes.

Molecular dynamics simulations of sequential cluster ion impacts

Molecular dynamics (MD) simulations of cluster and solid target collisions were performed in order to understand the relationship between surface deformation processes and cluster sizes. MD simulations of single impacts of clusters with various sizes showed that, when a cluster size is less than 10000 atoms, a crater-like damage could be caused at 20keV of total energy, while no damage was observed at Ar10000 and Ar20000 clusters. The surface morphology change was examined by MD of sequential irradiation of Ar clusters under various conditions. When the initial roughness of the surface was about 10Å in r.m.s. and the total incident energy was 20keV, the surface roughness was reduced with the impact of Ar13333 or larger cluster. The MD results give information about the final value and decay speed of surface roughness, which are required to optimize the cluster ion irradiation condition for various nano-scale modification processes.

Dynamics filter-concept and implementation of online motion generator for human figures

In this paper, we describe the concept and implementation of a dynamics filter, an online, full-body motion generator that converts a physically infeasible reference motion into a feasible one for the given human figure. Our implementation of the dynamics filter only uses time-local information, that is, does not require the whole motion sequence in advance. Therefore, the reference motion may be changed online in response to the interaction with a human or the environment. The dynamics filter is implemented based on an efficient rigid-body collision/contact model. This model itself provides an efficient algorithm for dynamics simulation of collisions and contacts. We demonstrate the power of the dynamics filter by several example motions that use motion capture data as a reference.

The Critical Exponent of Nuclear Fragmentation

Nuclei colliding at energies in the MeV’s break into fragments in a process that resembles a liquid-to-gas phase transition of the excited nuclear matter. If this is the case, phase changes occurring near the critical point should yield a “droplet” mass distribution of the form ≈A−T, with T (a critical exponent universal to many processes) within 2≤T≤3. This critical phenomenon, however, can be obscured by the finiteness in space of the nuclei and in time of the reaction. With this in mind, this work studies the possibility of having critical phenomena in small “static” systems (using percolation of cubic and spherical grids), and on small “dynamic” systems (using molecular dynamics simulations of nuclear collisions in two and three dimensions). This is done investigating the mass distributions produced by these models and extracting values of critical exponents. The specific conclusion is that the obtained values of T are within the range expected for critical phenomena, i.e. around 2.3, and the grand conclusion is that phase changes and critical phenomena appear to be possible in small and fast breaking systems, such as in collisions between heavy ions.

Direct Dynamics Simulations of Collision- and Surface-Induced Dissociation of N-Protonated Glycine. Shattering Fragmentation

Direct dynamics classical trajectory simulations are used to study energy transfer and unimolecular dissociation in collisions of N-protonated glycine, (gly-H)+, with an argon atom and a hydrogenated diamond {111} surface. The (gly-H)+ potential is represented by the AM1 semiempirical electronic structure theory and analytic potentials developed previously are used for the diamond surface and the (gly-H)+/Ar and (gly-H)+/diamond intermolecular potentials. The AM1 potential for (gly-H)+ gives the same collisional energy transfer distributions as does the AMBER empirical force field. For (gly-H)+ + diamond {111} at a collision energy and angle of 70 eV and 45°, the average percent energy transfer to (gly-H)+ vibration/rotation, to the surface, and to final ion translation are 12, 38, and 50, respectively. A distribution of (gly-H)+ dissociation products are observed in these collisions, with ∼55% of the dissociations occurring while (gly-H)+ collides with the surface, i.e., shattering fragmentation. Shatter...

Fusion and fragmentation of colliding metal clusters

In this paper, molecular dynamics simulations of collisions involving Ni n and Al n clusters are presented. A many-body potential based on tight-binding theory is used to model the interatomic interactions. The influence of several parameters such as cluster initial kinetic energy, size and type of atomic constituent is analyzed. At low energies, fusion reactions occur which are followed, at high bombarding energies, by multifragmentation events of the fused compound cluster. It is found that, for a given energy, the atomic structure and thermal properties of the compound cluster are very different for homo and heteroatomic collisions. We also obtain that the outcome of a heteroatomic collision is strongly affected by the nature of projectile and target atoms, specially in the case of unequally sized colliding clusters.

Computational studies of gas–carbon nanotube collision dynamics

Molecular dynamics simulations of collisions between gas particles and carbon nanotubes at elevated temperatures show that low frequency thermal vibrations of the nanotube dominate the scattering dynamics and that particle–tube energy transfer in the tube radial direction is far more efficient than that in the axial direction. The dominant role of the tube's thermal fluctuations reduces the importance of other collision parameters such as collider mass and collider–tube interaction strengths, and xenon, neon, helium and hydrogen particles all have similar collision dynamics and high scattering probabilities. At 1300 K, which is the temperature used in catalytic chemical vapour deposition single-walled nanotube growth, hydrogen molecules scatter from the tube while carbon atoms are efficiently trapped on the tube surface.

Collisional deactivation of CF 3I – a molecular dynamics simulation

The detailed mechanisms of ro-vibrational energy transfer in collisions between CF 3I and argon or propane are investigated. Molecular dynamics simulations of collisions between a reactant CF 3I molecule at energies from 50 to 200 kJ/mol with medium argon or propane at selected initial temperatures are interpreted in terms of ergodic collision limits. The intramolecular potential used for CF 3I is a Morse-stretch/harmonic-bend type function with parameters fitted to equilibrium structure, normal mode frequencies and dissociation energies. Simple generic Buckingham type pair-potentials are used for intermolecular atom–atom interactions. Energy transfer is related to (i) geometry of collision, (ii) impact parameter, (iii) number of atom–atom encounters, (iv) average dynamical hardness of interaction at atom–atom collisions, (v) number of minima in the center of mass separation and (vi) lifetime of the collisional complex. The energy transfer in our molecular dynamics calculations is compared with experimental results for the same colliders. The observed trends are interpreted in terms of detailed collisional mechanisms. Our results highlight the importance of rotational excitation and the repulsive part of the intermolecular potential.

Simulations of energetic beam deposition: From picoseconds to seconds

We present a new method for simulating crystal growth by energetic beam deposition. The method combines a Kinetic Monte-Carlo simulation for the thermal surface diffusion with a small scale molecular dynamics simulation of every single deposition event. We have implemented the method using the effective medium theory as a model potential for the atomic interactions, and present simulations for Ag/Ag(111) and Pt/Pt(111) for incoming energies up to 35 eV. The method is capable of following the growth of several monolayers at realistic growth rates of 1 monolayer per second, correctly accounting for both energy-induced atomic mobility and thermal surface diffusion. We find that the energy influences island and step densities and can induce layer-by-layer growth. We find an optimal energy for layer-by-layer growth (25 eV for Ag), which correlates with where the net impact-induced downward interlayer transport is at a maximum. A high step density is needed for energy induced layer-by-layer growth, hence the effect dies away at increased temperatures, where thermal surface diffusion reduces the step density. As part of the development of the method, we present molecular dynamics simulations of single atom-surface collisions on flat parts of the surface and near straight steps, we identify microscopic mechanisms by which the energy influences the growth, and we discuss the nature of the energy-induced atomic mobility.

Molecular dynamics simulations of binary collisions between water clusters

Molecular dynamics simulations of binary collisions between water clusters

The mechanism of energy transfer in H 2O–H 2O collisions – a molecular dynamics simulation

Earlier work on the activation-deactivation mechanism of gas phase unimolecular reactions is extended to the study of the detailed energy transfer mechanism in collisions of water molecules. Molecular dynamics simulations of binary collisions between a reactant water molecule at high internal energy with medium molecules at various selected initial temperatures are compared with results from approximate statistical theory. Energy transfer is related to (i) interaction strength, (ii) hard atom–atom encounters, (iii) multiple minima in the center of mass separation, (iv) collision lifetime and (v) anharmonicity of the intramolecular potential function. The observed trends are interpreted within the framework of the partially ergodic multiple encounter theory (PEMET) of collisional energy transfer. By comparison with typical stable molecule collisions the water–water collisions are more efficient as a reflection of the strong hydrogen bonding interactions. A good agreement between PEMET and molecular dynamics simulations over a wide range of interaction strengths and initial reactant energies is shown, indicating the possibility of a priori use of the PEMET model.

Polymerization via Cluster−Solid Surface Impacts: Molecular Dynamics Simulations

Molecular dynamics simulations of collisions between simple organic molecular clusters and a nonrigid, hydrogen-terminated diamond (111) surface have been performed to study the chemical reactions that occur within the clusters and between the cluster and the surface as a result of impact. A second-generation version of the reactive empirical bond order potential for hydrocarbons developed by Brenner, which has been modified to include long-range van der Waals interactions, is used in the simulations. The velocities considered are in the hyperthermal region and are comparable to those that result in shock-induced chemistry in energetic materials and that occur between particles and solid surfaces in interstellar space. The outcome of the scattering event is monitored as the reactivity of the molecular cluster is varied, and the dependence of the chemistry on the incident cluster velocity and the position of the reacting molecules within the cluster is examined. We find that polymerization reactions occur when the incident cluster velocity corresponds to an external kinetic energy that is within 3 eV/molecule of the binding energy of an individual cluster molecule. Following impact, some of the chemical products chemisorb to the surface in the initial stages of thin film growth, but the exact nature of the thin film is not clear.

Surface smoothing with energetic cluster beams

Impacts of energetic clusters, consisting of hundreds or thousands of atoms on a solid surface, have some remarkable effects, one of which is surface smoothing. To understand its mechanism, we have examined the results of molecular dynamics (MDs) simulations of cluster collisions with a solid which revealed that cluster impacts form craters on the surface and that ejected atoms have a significant lateral momentum component (parallel to the surface). It is postulated that these energetic atoms which are not in thermal equilibrium with the surface are responsible for rapid surface diffusion and consequently the surface smoothing. A new model of surface smoothing by energetic clusters has been developed. The model is based on nonlinear dynamics of a surface profile described by the noisy Kuramoto–Sivashinsky equation. A Monte Carlo procedure is used for simulating cluster impacts by distributing on a surface craters whose volume is defined by MDs simulations. The results of calculations show a qualitative agreement with experimental data on smoothing of Cu surface with 20 kV Ar clusters of 1000 atoms.

Solving the Langevin equation with stochastic algebraically correlated noise

The long time tail in the velocity and force autocorrelation function has been found recently in molecular dynamics simulations of peripheral collisions of ions. Simulation of those slowly decaying correlations in the stochastic transport theory requires the development of new methods of generating stochastic force of arbitrarily long correlation times. In this paper we propose a Markovian process, the multidimensional kangaroo process, which permits the description of various algebraically correlated stochastic processes.

Molecular dynamics simulations of low energy atomic collisions between an atom and a substrate: Effect of incident angle and energy

Atomic collisions during the ion beam deposition of Al thin films are simulated by a molecular dynamics method, assuming that the incident atoms are neutralized. The kinetic energies of the incident atoms range from 10 to 50 eV and the angles of incident to Al substrate are controlled between π 6 and π 3 . The simulation results show reflection or rapid trapping of the incident atoms as well as a surface migration for a longer distance. As a response to the collision, atoms in the substrate show highly directional or much more diffused propagation of thermal vibration, depending on the incident angle or timing of the collision.

Cluster Growth from the Gas Phase: Associative Collisions of Small Alkali Halide Aggregates

Growth of clusters and eventually particles from free molecules in the gas phase lends itself to simulation.This report discusses classical molecular dynamics simulations of collisions of KCl molecules with small KCl clusters and of small KCl clusters with each other. Particular attention is given to long-lived collision complexes, especially those that endure long enough to be models for association and growth. Mechanisms of breakup are examined, as to structures through which the complexes go and the exchange processes that appear.

Geometry-Dependent Trends in the Scattering of van der Waals Clusters from Crystal Surfaces

Constrained configurations in linear and ring geometries along with a polyhedral structure of van der Waals clusters were used to observe classical molecular dynamics simulations of collisions of these clusters with a rigid surface. The constrained geometries tend to survive high-energy collisions intact while the minimized polyhedral structures tend to shatter. The constrained structures also tend to scatter nearer the specular angle. 36 refs., 9 figs., 1 tab.

Molecular dynamics simulations of cluster collisions

Molecular dynamics simulations are very powerful to get more insight into the dynamics of collision processes. Simulations of such processes have been successfully performed using empirical interaction potentials. More recently the combination of the Density-Functional-Theory within the Local Density Approximation (DFT-LDA) with Molecular Dynamics (MD) has been realized. A simplified LCAO-DFT-LDA scheme, which enables to consider the electronic states in the calculation of the forces on the atoms, was developed and applied for simulations of molecule-cluster and cluster-cluster collisions. In the present paper the authors demonstrate the extension of such calculations under the consideration of the statistics in cluster collision processes by using parallel computer techniques.

Molecular Dynamics Simulations of Hypervelocity Buckminsterfullerene Collisions

ABSTRACTMolecular dynamics simulations of high-energy collisions between various combinations of C60 and C70 fullerenes were performed to calculate the threshold for molecular fusion of these clusters as a function of the center-of-mass collision energy. For collision energies below 90 eV, only non-reacting collisions occurred with no observation of any fusion. However, at higher collision energies molecular fusion of the colliding clusters was observed with the fusion probability approaching 1 by 160 eV collision energy. The non-fusing, rebounding collisions showed deeply inelastic behavior with the loss of translational energy to internal energy varying from 50 to 70 percent.