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[en] Nanosprings have drawn continuous attention due to their superior elongation and potential applications in stretchable devices. Based on molecular dynamics (MD) simulations, the deformation mechanism of Cu nanosprings with/without twin boundary (TB) structures is investigated in this work. It is found that dislocation-driven deformation mechanism of nanosprings mainly depends on their geometry parameters. During the plastic process, severe distortion caused by local dislocation emission is frequently observed, especially for nanosprings with large wire diameters. Small twin boundary spacings (TBSs) can effectively improve the mechanical properties of nanosprings through restricting dislocation emission and TB migration. In addition, the calculated spring constant reveals that the stiffness of nanosprings with larger wire diameters, smaller helix pitches or smaller TBSs will become larger. It is also worth mentioning that the classical theory is still valid in nanosprings with TB structures. These findings open a new avenue to design novel nanosprings for nanodevices. (paper)
[en] The angular dependence of the sticking coefficient of CHx projectiles on a prototypical amorphous hydrocarbon surface was studied using molecular dynamics. The resulting datasets are fitted to appropriate fit formulas to allow interpolation and lookup of sticking coefficients at arbitrary parameter values.
[en] Highlights: • Film heterogeneity manifests in overall and local volume, energy and dynamic. • Solid and free interfaces foster asynchronous glass transition of the nanofilm. • Percolation of immobile domains relates glass transition to film heterogeneity. • Immobile domains initiate and accelerate film percolation during glass transition.
[en] How to properly account for polyvalent counterions in a molecular dynamics simulation of polyelectrolytes such as nucleic acids remains an open question. Not only do counterions such as Mg2+ screen electrostatic interactions, they also produce attractive intrachain interactions that stabilize secondary and tertiary structures. Here, we show how a simple force field derived from a recently reported implicit counterion model can be integrated into a molecular dynamics simulation for RNAs to realistically reproduce key structural details of both single-stranded and base-paired RNA constructs. This divalent counterion model is computationally efficient. It works with existing atomistic force fields, or coarse-grained models may be tuned to work with it. We provide optimized parameters for a coarse-grained RNA model that takes advantage of this new counterion force field. Using the new model, we illustrate how the structural flexibility of RNA two-way junctions is modified under different salt conditions.
[en] We explore the atomic origins of the structural phase transformations (PTs) in AlxCrCoFeNi high entropy alloy (HEA) using classical molecular dynamics (MD) simulations. Our investigation critically reveals the role of Al content in triggering such diffusive transformations from a molten to a crystalline phase (for lower Al concentrations) or from molten to amorphous transitions (for Al fractions above the equiatomic alloy composition). Structural pair-correlation functions employed to provide atomistic evidence and mechanisms for the PTs show that the molten to amorphous PT initiates through the nucleation of a final child phase in the parent molten phase. Our structure predictions, although differ from earlier experimental observations, are confirmed by the predictions from common-neighbor analysis.
[en] Molecular dynamic simulations were performed to examine the wetting behavior of a graphite surface textured with nanoscale pillars. The contact angle of a water droplet on parallelepiped or dome-shaped pillars was investigated by systematically varying the height and width of the pillar and the spacing between pillars. An optimal inter-pillar spacing that gives the highest contact angle was found. The droplet on the dome-covered surface was determined to be more mobile than that on the surface covered with parallelepiped pillars
[en] The calculation of free energy differences for thermally activated mechanisms in the solid state are routinely hindered by the inability to define a set of collective variable functions that accurately describe the mechanism under study. Even when possible, the requirement of descriptors for each mechanism under study prevents implementation of free energy calculations in the growing range of automated material simulation schemes. We provide a solution, deriving a path-based, exact expression for free energy differences in the solid state which does not require a converged reaction pathway, collective variable functions, Gram matrix evaluations, or probability flux-based estimators. The generality and efficiency of our method is demonstrated on a complex transformation of C15 interstitial defects in iron and double kink nucleation on a screw dislocation in tungsten, the latter system consisting of more than 120 000 atoms. Both cases exhibit significant anharmonicity under experimentally relevant temperatures. (authors)
[en] We study the multi-fragmentation for the different parameterizations of the density dependent symmetry energy using an isospin-dependent quantum molecular dynamics (IQMD) model. We analyze the sensitivity of fragment production towards various forms of the density dependent symmetry energy. The inclusion of momentum dependent interactions (MDI) results in a larger variation of fragment production. We here highlighted the collective response of the MDI and symmetry energy towards the fragmentation of colliding nuclei at intermediate energies.
[en] The synthetic driving force method is a widely-used technique in molecular dynamics simulations to investigate the migration of grain boundaries. Its physical essence, however, has been under debate for quite some time for generating the driving force by artificially introducing some energy into the crystals. In this study, the elementary process governing the grain boundary motion under the driven motion method was explored by applying a varying synthetic driving force that increases from zero at a constant rate, which is in contrast to a constant driving force that is usually applied in past studies. With this method, it was found that a rate-controlling process, i.e., disconnection nucleation that has been reported before to dominate the physical grain boundary motion coupled to an applied shear, also operated for grain boundary motion caused by the synthetic driving force. Furthermore, the disconnection nucleation mediated process was also found to cause a strong size dependence and transitions of grain boundary motion modes at different temperatures. It is hoped that with this study, the synthetic driving force method in studying grain boundary motion can be used with more confidence in its physical essence and a universal mechanism can be proposed to explain grain boundary motion in materials despite how it is caused.