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Large-scale nonequilibrium molecular dynamics simulation is used to study the effect of grain size and Cr segregation at the twin boundary (TB) on the spalling fracture mechanism of nickel based alloys. In particular, loading waves are designed so that the maximum tensile stress first appears in the grain interior for all the crystals with different grain sizes. In contrast to traditional understandings, no monotonous relationship between the spall strength and the grain size appears in our results. The spall strength is found to depend on the wave attenuation distance measured from the first maximum tensile stress position to the spalled TB as well as the accompanied microstructure evolutions. The number of spalling plane increases with the increase of TB or the decrease of the grain size. As the grain size continues to decrease, a greater impact strength is required to cause spallation fracture at multiple TBs. In this case, the spall strength becomes insensitive to the first maximum tensile stress position. With the increment of solute atom concentration, the number of the spalling plane increases when the solution is segregated. But it decreases when the solution is uniformly distributed. Such a result is explained by segregation-enhanced energy dissipation and interactions between the waves and the microstructures nearby TBs. In particular, the shock wave would induce a local lattice reorientation nearby the TB depending on its segregation degree and the lattice reorientation would modify the slip manner of stacking faults and, thus, affect void nucleation and growth. The lattice reorientation would also contribute to the generation of sub-grain boundaries inside the grains in terms of the movement of stacking faults. Void nucleation at the sub-GB is the main cause of fracture at the grain interior. [ABSTRACT FROM AUTHOR]

The bimolecular reaction between OH− and CH3F is not just a prototypical SN2 process, but it has three other product channels. Here, we develop an accurate full-dimensional potential energy surface (PES) based on 191 193 points calculated at the level CCSD(T)-F12a/aug-cc-pVTZ. A detailed dynamics and mechanism analysis was carried out on this potential energy surface using the quasi-classical trajectory approach. It is verified that the trajectories do not follow the minimum energy path (MEP), but directly dissociate to F− and CH3OH. In addition, a new transition state for proton exchange and a new product complex CH2F−⋯H2O for proton abstraction were discovered. The trajectories avoid the transition state or this complex, instead dissociate to H2O and CH2F− directly through the ridge regions of the minimum energy path before the transition state. These non-MEP dynamics become more pronounced at high collision energies. Detailed dynamic simulations provide new insights into the atomic-level mechanisms of the title reaction, thanks to the new chemically accurate PES, with the aid of machine learning. [ABSTRACT FROM AUTHOR]

The size dependence of the surface tension of critical bubbles in a superheated (stretched) Lennard–Jones solution with complete solubility of the components is considered. Two approaches are used to determine this dependence. The first one is based on the van der Waals gradient theory, and the second one is based on molecular dynamic simulation results of nucleation in a solution. It is established that, unlike in a one-component liquid, where the surface tension of the equilibrium bubble is less than that for the flat interface, in solution, it can exceed the flat limit. The ranges of temperatures, pressures, and mixture compositions, where this effect occurs, are determined. The asymptotic behavior of the surface tension of vapor phase nuclei within the limits of zero and infinitely large curvature of the dividing surface is analyzed. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulation, we studied thermodynamic effects of a nanobubble's collapse-induced erosion occurring at different ambient temperatures. We analyzed the dynamics of a single nanobubble collapsing near an aluminum (Al) solid boundary immersed in water at temperatures ranging 10–60 °C (283 – 333 K). We used a momentum mirror protocol to investigate the nanobubble's collapse-induced shock wave as the associated nanojet formed and moved toward the solid boundary. The results showed that the nanojet was formed during the collapse process after the collision of the nanobubble with the shock wave. On the aluminum surface, the erosion at lower ambient temperatures was greater than at higher ambient temperatures. [ABSTRACT FROM AUTHOR]

The non-radiative relaxation process within the Q-bands of chlorophylls represents a crucial preliminary step during the photosynthetic mechanism. Despite several experimental and theoretical efforts performed in order to clarify the complex dynamics characterizing this stage, a complete understanding of this mechanism is still far to be reached. In this study, non-adiabatic excited-state molecular dynamic simulations have been performed to model the non-radiative process within the Q-bands for a model system of chlorophylls. This system has been considered in the gas phase and then, to have a more representative picture of the environment, with implicit and mixed implicit–explicit solvation models. In the first part of this analysis, absorption spectra have been simulated for each model in order to guide the setup for the non-adiabatic excited-state molecular dynamic simulations. Then, non-adiabatic excited-state molecular dynamic simulations have been performed on a large set of independent trajectories and the population of the Qx and Qy states has been computed as the average of all the trajectories, estimating the rate constant for the process. Finally, with the aim of investigating the possible role played by the solvent in the Qx–Qy crossing mechanism, an essential dynamic analysis has been performed on the generated data, allowing one to find the most important motions during the simulated dynamics. [ABSTRACT FROM AUTHOR]

The transport behavior of ionic liquids (ILs) is pivotal for a variety of applications, especially when ILs are used as electrolytes. Many aspects of the transport dynamics of ILs remain to be understood. Here, a common ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf2), was studied with molecular dynamics simulations. The results show that BmimNTf2 displays typical structural relaxation, subdiffusive behavior, and a breakdown of the Stokes-Einstein diffusion relation as in glass-forming liquids. In addition, the simulations show that the translational dynamics, reorientation dynamics, and structural relaxation dynamics are well described by the Vogel-Fulcher-Tammann equation like fragile glass forming liquids. Building on previous work that employed ion cage models, it was found that the diffusion dynamics of the cations and anions were well described by a hopping process random walk where the step time is the ion cage lifetime obtained from the cage correlation function. Detailed analysis of the ion cage structures indicated that the electrostatic potential energy of the ion cage dominates the diffusion dynamics of the caged ion. The ion orientational relaxation dynamics showed that ion reorientation is a necessary step for ion cage restructuring. The dynamic ion cage model description of ion diffusion presented here may have implications for designing ILs to control their transport behavior. [ABSTRACT FROM AUTHOR]

In the current research, we studied the collapse mechanism of the nanobubble under mirror and real wall protocols using molecular dynamics simulation. Moreover, we analyzed reactive properties of the real wall during the collapse process. Towards this aim, an aluminum (Al) slab has been considered as a real wall, and its behavior after the formation of a nanojet has been investigated. The obtained results indicated that the dynamics of nanobubble collapse under the mirror and real protocols are similar. The collision between the shock wave and the nanobubble leads to the collapse of nanobubble, and the nanojet was formed during this collapse process. However, the nanobubble can collapse sooner when the Al slab is used as a real wall. Moreover, the surface roughness of the Al slab during the shock propagation and nanobubble collapse was increased due to the formation of the chemical reaction between Al and water under the real wall protocol, while the mirror wall has a roughness-free surface from the beginning to the end. The chemical reaction 2Al + H 2 O → AlOH + AlH creates the amorphous layer containing the AlOH and AlH species on the surface of the Al slab. This layer grows semi-smoothly during bubble shrinkage and collapse process, while the growth type was changed to an island shape after the complete collapsing. The island shape on the Al slab was formed behind the nanojet due to the water vortices that are formed after the nanobubble collapse. [ABSTRACT FROM AUTHOR]

Modern particle-based simulations increasingly incorporate polarization charges arising from spatially nonuniform permittivity. For complex dielectric geometries, calculation of these induced many-body effects typically requires numerical solvers based upon boundary-element methods, which very significantly increase the required computational effort. For the special case of dielectric spheres, such as colloids or nanoparticles, we recently proposed a semianalytical spectrally accurate hybrid method that combines the method of moments, the image-charge method, and the fast multipole method. The hybrid method is efficient and accurate even when dielectric spheres are closely packed. Here, we extend the method to the evaluation of direct and induced electrostatic forces and demonstrate how this can be incorporated in molecular dynamics simulations. The choice of the relevant numerical parameters for molecular dynamics simulations is discussed in detail, as well as comparisons to the boundary-element method. As a concrete example, we examine the challenging case of binary crystal structures composed of close-packed dielectric colloidal spheres. [ABSTRACT FROM AUTHOR]

We use molecular dynamic simulations to investigate the motion of a folding molecular motor inside soft matter. Purcell's scallop theorem forbids the displacement of the motor due to time symmetrical hydrodynamic laws at low Reynolds numbers whatever the asymmetry of the folding and unfolding rates. However, the fluctuation theorems imply a violation of the time symmetry of the motor's trajectories due to the entropy generated by the motor, suggesting a breakdown of the scallop theorem at the nanoscale. To clarify this picture, we study the predicted violation of time reversibility of the motor's trajectories, using two reverse asymmetric folding mechanisms. We actually observe this violation of time reversibility of the motor's trajectories. We also observe the previously reported fluidization of the medium induced by the motor's folding, but find that this induced diffusion is not enough to explain the increase of the motor's displacement. As a result, the motor is not carried by the medium in our system but moves by its own, in violation of the scallop theorem. The observed violation of the scallop theorem opens a route to create very simple molecular motors moving in soft matter environments. [ABSTRACT FROM AUTHOR]

This study aims to provide new insights into the influencing factors of the Earth (low temperature, depth, and annealing time) on the heterogeneous kinetics of bulk Fe2O3 by the molecular dynamics simulation method. The obtained results show that there is an influence of the low temperature corresponding to the temperature of liquefied gases, such as helium (4.212 K), nitrogen (77 K), argon (83.8058 K), oxygen (90 K), and carbon (194.5 K), the depth (h) of the Earth's surface from h0 = 0 km to h5* = 6370 km that corresponds to the temperature (T) from T = 300 K to T = 7000 K and the pressure (P) from P = 0 GPa to P = 360 GPa, and then annealing time (t) (120 ps) on the heterogeneous kinetics of bulk Fe2O3, such as the Radial Distribution Function (RDF), Coordination Number (CN), angular distribution, number of structural units, size (l), and energy (E). When the temperature increases in the low temperature (T) region at zero pressure (P), the link length (r), RDF height, size, CN, and the number of structural units FeO4, FeO5, and FeO6 do not change significantly, but only the very large change in E serves as the basis for future research on the mechanical properties and electrical conductivity of semiconductor materials. When the depth (h) of the Earth's surface and the thermal annealing time at different locations are increased, the characteristic quantities of dynamic dynamics change greatly, including the disappearance of FeO4 at depth h1 ≥ 17.5 km and the appearance of additional structural units FeO7, FeO8, and FeO9 at h3 ≥ 1742 km and FeO10 at h5 ≥ 5562.5 km. [ABSTRACT FROM AUTHOR]

Fundamental understanding of water confined in graphene is crucial to optimally design and operate sustainable energy, water desalination, and bio-medical systems. However, the current understanding predominantly remains in the static properties near the graphene surfaces. In this paper, a key water transport property, i.e., self-diffusivity, is examined under confinement by various graphene nanogap sizes (Lz = 0.7-4.17nm), using molecular dynamics simulations with various graphene-water interatomic potentials (Simple Point Charge (SPC/E) and TIP3P water models). It is found that the water self-diffusivity nearly linearly decreases as the graphene-water interatomic potential energy increases at a given nanogap size. It also decreases as the graphene nanogap size decreases down to Lz = 1.34 nm; however, it shows the peak water self-diffusivity at Lz = 0.8 nm and then continues to decrease. The peak water self-diffusivity is related to the significant change of the overlapping surface force, and associated, nonlinear local water density distribution. The in-plane water selfdiffusivity is higher up to nearly an order of magnitude than that of the out-of-plane due to the geometrical confinement effect by the graphene nanogap. The obtained results provide a roadmap to fundamentally understand the water transport properties in the graphene geometries and surface interactions. [ABSTRACT FROM AUTHOR]

The ability to quantitatively predict and analyze the rate of electron spin relaxation of open-shell systems is important for electron paramagnetic resonance and paramagnetic nuclear magnetic resonance spectroscopies. We present a combined molecular dynamics (MD), quantum chemistry (QC), and spin dynamics simulation method for calculating such spin relaxation rates. The method is based on the sampling of a MD trajectory by QC calculations, to produce instantaneous parameters of the spin Hamiltonian used, in turn, to numerically solve the Liouville-von Neumann equation for the time evolution of the spin density matrix. We demonstrate the approach by simulating the relaxation of electron spin in an aqueous solution of Ni2+ ion. The spin-lattice (T1) and spin-spin (T2) relaxation rates are extracted directly from the simulations of the time dependence of the longitudinal and transverse magnetization, respectively. Good agreement with the available, indirectly obtained experimental data is obtained by our method. [ABSTRACT FROM AUTHOR]

Molecular dynamics methods are used to investigate the effect of wall wettability and initial fluid density on the nucleation and growth of nanobubbles in a nanochannel with a groove. Surface wettability is controlled by adjusting the parameters of the solid-liquid potential energy and the results show that groove wettability greatly influences the bubble nucleation process. In the case of a groove with a weak solid-liquid interaction, fluid atoms in the groove are strongly affected by repulsive forces attributed to solid atoms, the density of the fluid atoms is low, and atomic collision frequency increases. The local activation energy is concentrated in the groove, resulting in bubble nucleation. In the case of a groove with a stronger solid-liquid interaction, the bubble is formed in the center of the nanochannel. In general, the bubble radius of curvature and contact angle remain unchanged during the growth process in the case of a uniform wettability nanochannel with a groove, while significant changes are observed for a nonuniform wettability nanochannel. The radius of curvature for the generated bubble increases with increasing fluid initial density, while the steady contact angle decreases. [ABSTRACT FROM AUTHOR]

Large-scale classical molecular dynamics simulations of epitaxial TiN/TiN(001) thin film growth at 1200K are carried out using incident flux ratios N/Ti=1, 2, and 4. The films are analyzed as a function of composition, island size distribution, island edge orientation, and vacancy formation. Results show that N/Ti=1 films are globally understoichiometric with dispersed Ti-rich surface regions which serve as traps to nucleate 111-oriented islands, leading to local epitaxial breakdown. Films grown with N/Ti=2 are approximately stoichiometric and the growth mode is closer to layer-by-layer, while N/Ti=4 films are stoichiometric with N-rich surfaces. As N/Ti is increased from 1 to 4, island edges are increasingly polar, i.e., 110-oriented, and N-terminated to accommodate the excess N flux, some of which is lost by reflection of incident N atoms. N vacancies are produced in the surface layer during film deposition with N/Ti=1 due to the formation and subsequent desorption of N2 molecules composed of a N adatom and a N surface atom, as well as itinerant Ti adatoms pulling up N surface atoms. The N vacancy concentration is significantly reduced as N/Ti is increased to 2; with N/Ti=4, Ti vacancies dominate. Overall, our results show that an insufficient N/Ti ratio leads to surface roughening via nucleation of small dispersed 111 islands, whereas high N/Ti ratios result in surface roughening due to more rapid upper-layer nucleation and mound formation. The growth mode of N/Ti=2 films, which have smoother surfaces, is closer to layer-by-layer. [ABSTRACT FROM AUTHOR]

Impact of energetic nanoparticles on solids produces craters on the surface. We use molecular dynamics simulations to compare crater production on a compact Au solid with that in a porous (foam) target. Our results show a complex picture: (i) At low impact velocities, the nanoparticles produce permanent craters in the foam while they cannot penetrate the compact target. (ii) With increasing velocity and/or projectile mass, the crater depth in the foam target increases less strongly than for the compact target. The plasticity-affected zone in the foam target is of similar size as in the compact target. Our results are relevant for the use of porous structures as shields against nanoparticle impact. [ABSTRACT FROM AUTHOR]