Simple Lennard-Jones Potential Research Articles

Edge effects on adsorption of Lennard-Jones fluid in finite carbon slits

Abstract The grand canonical Monte Carlo (GCMC) simulation method is used to study adsorption of methane in finite carbon slits. Simple Lennard-Jones potential is used to describe intermolecular interactions for methane. Slits between two identical carbon cylinders and between a cylinder and a planar infinite solid surface are considered. Adsorption field non-uniformity generated by edges leads to shifts of adsorption isotherms and to changes of the fluid local structure in pores. The enhancement of edge effects at larger distances from the wall is demonstrated. The behavior of the fluid density in finite slits predicted from the asymptotic theory has been confirmed.

Simulations of cold nuclear matter at sub-saturation densities

Ideal nuclear matter is expected to undergo a first order phase transition at the thermodynamic limit. At such phase transitions the size of density fluctuations (bubbles or droplets) scale with the size of the system. This means that simulations of nuclear matter at sub-saturation densities will inexorably suffer from what is vaguely referred to as “finite size effects”. It is usually thought that these finite size effects can be diminished by imposing periodic boundary conditions and making the system large enough, but as we show in this work, that is actually not the case at sub-saturation densities. In this paper we analyze the equilibrium configurations of molecular dynamics simulations of a classical model for symmetric ideal (uncharged) nuclear matter at sub-saturation densities and low temperatures, where phase coexistence is expected at the thermodynamic limit. We show that the most stable configurations in this density range are almost completely determined by artificial aspects of the simulations (i.e. boundary conditions) and can be predicted analytically by surface minimization. This result is very general and is shown to hold true for several well known semi-classical models of nuclear interaction and even for a simple Lennard-Jones potential. Also, in the limit of very large systems, when “small size” effects can be neglected, those equilibrium configurations seem to be restricted to a few structures reminiscent to the “Pasta Phases” expected in Neutron Star matter, but arising from a completely different origin: In Neutron Star matter, the non-homogeneous structures arise from a competition between nuclear and Coulomb interactions while for ideal nuclear matter they emerge from finite (yet not “small”) size effects. The role of periodic boundary conditions and finite size effects in Neutron Star matter simulations are reexamined.

Molecular Simulation Approach to the Effects of Additives in Electrodeposition Process

The leveling effect of additives in electrodeposition has been studied using a simplified model for inhibitors by the combined method of molecular dynamics and Monte Carlo simulations. The additive molecule is assumed to be a spherical particle which has an action range around its center in which the metal deposition is inhibited. The interaction between the additives and the electrode is modeled by a simple Lennard-Jones potential. The dependence of the leveling efficiency on the action range and the interaction energy has been studied by a series of simulations of filling grooves. It is shown that high throwing power is obtained for the additives with large interaction energy with the electrode.

Large-scale molecular dynamics simulation of sputtering process with glancing-angle Ar cluster impact on 4H-SiC

Large-scale molecular dynamics simulations with two Ar688 cluster impacts on a 4H-SiC surface are performed to investigate the mechanism of lateral sputtering caused by two clusters collisions. The two Ar clusters are composed of 688 atoms each (referred as Ar688) which are described by a simple Lennard–Jones potential. The initial velocities of both clusters are 2.55×104m/s when the acceleration voltage is 100keV. The computational volume is 30nm×30nm×16nm, which is constructed by 1444608 4H-SiC atoms. At 0.8ps after the impact from the first Ar cluster on the 4H-SiC surface, a second argon cluster with predetermined incident-angle collides with 4H-SiC surface at a distance of one “diameter” away from the center of the first impact where the term “diameter” refers to the diameter of the footprint of the first impact on 4H-SiC. The incident-angle of the second argon cluster was set at 0°, 60°, or 80° for three different trials. Consequently, in each case the crater formed by the first cluster showed signs of being smeared out by the impact of the second cluster. Especially at the incident-angle of 80° the effects of surface modification were clearly noticeable.

Molecular dynamics simulation of nucleation of KBr clusters confined within armchair single-walled carbon nanotubes

We carry out molecular dynamics (MD) simulation studies on the phase transition and nucleation of (KBr) N clusters confined within armchair single-walled carbon nanotubes ((R,R)-SWNTs) with Born–Mayer–Huggins’ potential functions for the ion–ion interactions and simple Lennard-Jones potentials for the ion–carbon interactions. A novel crystallite of KBr cluster inside (R,R)-SWNT is observed during slow cooling or quenching runs. Some important thermodynamic and dynamic parameters are estimated. The effect of (R,R)-SWNT on nucleation and phase transition of KBr clusters is analyzed and discussed.

Negative Poisson’s Ratio Materials via Isotropic Interactions

We show that under tension a classical many-body system with only isotropic pair interactions in a crystalline state can, counterintuitively, have a negative Poisson's ratio, or auxetic behavior. We derive the conditions under which the triangular lattice in two dimensions and lattices with cubic symmetry in three dimensions exhibit a negative Poisson's ratio. In the former case, the simple Lennard-Jones potential can give rise to auxetic behavior. In the latter case, a negative Poisson's ratio can be exhibited even when the material is constrained to be elastically isotropic.

A method of accelerating molecular dynamics simulation for atomic diffusion in metallic interface

Many interesting long-time dynamic properties of solid interface, such as deep diffusion, pervasion and phase forming, cannot be simulated directly using traditional molecular dynamics (MD) because of nanosecond timescale limitations. Thus, a simpler bias potential form has been proposed within the Voter's hyper dynamics scheme. In this method, the potential energy wells are raised by adding a coefficient, which was defined as the accelerating factor, to the original potential. So, the escape rate from potential wells was enhanced, which extends the timescale by several orders of magnitude comparing to the traditional MD simulations. What's more important, the features of potential surface are reserved even without any in_advance knowledge of the location of either the potential energy wells or saddle points. We demonstrate this method by applying it to the mutual diffusion of atoms in Mg/Zn interface with different accelerating factors using a simple Lennard-Jones potential. The results showed that long-time MD simulation can be realized very easily by our approach.

Predictions of Transport Properties in Gaseous Mixtures of Sulfur Hexafluoride and Nitrogen

We present and discuss here results concerning the implementation of molecular dynamics simulations with the Green-Kubo formalism to predict transport coefficients in SF 6 /N 2 mixtures under conditions of practical interest. SF 6 was modeled using a flexible molecular force field recently proposed [Olivet, A.; Vega, L. F. J. Chem. Phys. 2007, 126, 144502], while the force field of Galassi and Tildesley was used for N 2 modeling [Galassi, G.; Tildesley, D. J. Mol. Simul. 1994, 13, 11]. The influence of mixture composition on the mutual diffusion coefficients and shear viscosities was investigated by a series of molecular dynamic simulations of SF 6 /N 2 mixtures with different compositions at 300 K and 1 MPa. Temperature and pressure dependencies of the mutual diffusion coefficient were also investigated for a fixed composition of SF 6 /N 2 , covering the temperature range from 260 to 340 K and two pressure values (1 and 2 MPa). Simulations with these anisotropic force fields led to mutual diffusion coefficients that diminish when the SF 6 concentration in the mixture is increased. This behavior differs from the hypothesis of no composition dependence assumed for several estimation methods available in the literature for gaseous binary mixtures at low pressures. Such a composition dependence is explained on the basis of the deviations from ideality of SF 6 /N 2 mixtures. The effect of the molecular flexibility considered in the SF 6 molecular model is analyzed by comparing the transport coefficients estimated with the aforementioned force fields to those estimated with simple Lennard-Jones potentials. Differences between predictions with the anisotropic force fields and the simpler Lennard-Jones potentials were more significant in the case of the shear viscosity; while viscosities estimated with the Lennard-Jones potentials showed no dependence on composition, the estimations based on the anisotropic force fields exhibited a maximum value for the viscosity, which is the same behavior described by an empirical method. Results presented here are pure predictions, thus covering part of the lack of information existing about the transport coefficients of these mixtures, which is of relevance for several industrial processes.

Large-scale molecular dynamics simulation for two Ar clusters impact on 4H–SiC

Large-scale molecular dynamics simulations with two Ar688 cluster impacts on a 4H–SiC surface are performed to investigate two clusters collisions for lateral sputtering mechanism. The target atoms are described by Tersoff potential with some modifications for 4H–SiC. Two Ar clusters are assumed by Ar688, which are described by simple Lennard-Jones potential. The initial velocity of both clusters is 2.55×104m/s when the acceleration voltage is 100keV. The computational volume is 30nm×30nm×16nm, which is constructed by 1444608 4H–SiC atoms. After 0.8ps of the impact on the 4H–SiC surface from a first Ar cluster, a second argon cluster collides at a position that is one “diameter” away from the center of the first impact and where term the “diameter” refers to the diameter of the footprint of the first impact on 4H–SiC. Consequently, the crater formed by the first cluster is smeared out by the lateral sputtering effect of the second cluster.

材料界面における力学特性評価へのSPHとMDによるアプローチ(S09-2 計算ナノ・マイクロテクノロジーの新展開(2),S09 計算ナノ・マクロテクノロジーの新展開)

Scale difference of interface strength is discussed based on results obtained by two genuine particle methods. Microscopic method is molecular dynamics (MD) method, and another, microscopic one, is smoothed particle hydrodynamics (SPH) method. Our materials interface model is constructed as bi-material (aluminum, steel, or copper) composite with flat joint surrounded by free surfaces. In SPH analysis, three-dimensional elastic constitutive relation is built in, with employing two kinds of macroscopic materials constants. MD analysis is constructed by simple Lennard-Jones potential. In spite of difference in size, both particle methods can reproduce adequate characteristics of stress concentration along interface theoretically predicted in elastic regime. It is found that in SPH method tensile stress is transmitted with oscillation, and this will offer nice matching with molecular vibration in MD method.

Quantum chemical exploration of the HCl dimer interaction

Quantum chemical calculations based on the MP2 perturbation method with a large basis set are used to determine a pointwise representation of the HCl dimer interaction at equilibrium bondlengths. A series of intermolecular potentials are then fitted to the quantum chemical data with the aim of exploring the convergence in accuracy with increasing complexity in the functional forms. Beginning with simple Lennard-Jones potentials with electrostatics added we progress to more complex functional forms based on distributed multipole approximation and anisotropic representations of exchange repulsion and polarisation. Pitfalls in the fitting procedure are discussed, and convergence is monitored by examination of second virial coefficient data, the structure of the stationary points and spectroscopic parameters.

Temperature dependence of the levitation effect Implications for separation of multicomponent mixtures

Sufficiently long molecular dynamics simulations have been carried out on spherical monatomic sorbates in NaY zeolite, interacting via simple Lennard-Jones potentials, to investigate the dependence of the levitation effect on the temperature. Simulations carried out in the range 100–300 K suggest that the anomalous peak in the diffusion coefficient (observed when the levitation parameter, γ, is near unity) decreases in intensity with increase in temperature. The rate of cage-to-cage migrations also exhibits a similar trend. The activation energy obtained from Arrhenius plots is found to exhibit a minimum when the diffusion coefficient is a maximum, corresponding to the γ≈1 sorbate diameter. In the linear or normal regime, the activation energy increases with increase in sorbate diameter until it shows a sharp decrease in the anomalous regime. Locations and energies of the adsorption sites and their dependence on the sorbate size gives interesting insight into the nature of the underlying potential-energy surface and further explain the observed trend in the activation energy with sorbate size. Cage residence times, τc, show little or no change with temperature for the sorbate with diameter corresponding to γ≈1, whereas there is a significant decrease in τc with increase in temperature for sorbates in the linear regime. The implications of the present study for the separation of mixtures of sorbates are discussed.

On the elastic behavior of composition-modulated superlattices

Atomistic computer simulations are used to systematically investigate the role of interfacial disorder on the elastic behavior of composition-modulated superlattices of fcc metals, represented by simple Lennard–Jones potentials. The structures, energies, and average elastic properties of four types of superlattices with various degrees of interfacial disorder are computed as a function of the modulation wavelength along [001]. The four superlattice types studied include perfectly coherent, incoherent, and two types derived from these by introducing relative twists about [001] between alternating layers. A 20% lattice-parameter mismatch between the two modulating materials is assumed. Results are compared with our earlier work on unsupported thin films, grain-boundary superlattices, and incoherent superlattices with a 10% lattice-parameter mismatch. The degree of structural disorder at the interfaces is found to correlate well with the magnitude of the elastic anomalies, which cannot be accounted for by anisotropic lattice-parameter changes alone. The grain-boundary superlattices studied earlier are found to provide a good model limit for the elastic behavior of interfacially disordered dissimilar-material superlattices.

Diffusion and Glass Transition in Simple Liquids

The free volume theory of diffusion for hard spheres, earlier developed by Cohen and Turnbull, is modified for simple van der Waals type liquids. The modified theory fits the self-diffusion data for argon, krypton, and xenon fairly well and predicts glass transition for these liquids at approximately one-third of their normal boiling points. It is found that a more accurate model for the free volume diffusion in liquids should include a redistribution energy for voids arising from the nonlinear behavior of the pair potential. The agreement found by Cohen and Turnbull between their hard sphere theory and the self-diffusion data of some large organic molecules is interpreted as evidence that the simple Lennard-Jones potential does not accurately describe their interaction with each other. This situation is reflected in the failure of these molecules to obey the same corresponding states relationship as the simple liquids of argon, krypton, and xenon.