Molecular dynamics simulations of phase transition of n-nonadecane under high pressure

The phase transition behavior of n-nonadecane under high pressure was investigated with molecular dynamics (MD) simulations method. A simplified model with amorphous structure and periodic boundary conditions in constant-temperature, constant-pressure ensemble was used in this study. The results showed that the whirling and molecules motion of n-nonadecane chains were restrained by the high pressure. The simulated phase transition temperature of n-nonadecane under high pressure is higher than that under atmospheric pressure. The order parameter of n-nonadecane decreases with the increase in temperature. The simulations reveal that MD is an effective method to understand the phase transition of alkane-based phase change materials on molecular and atomic scale.     

Molecular dynamics simulation of diffusionless phase transformation in a quenched NiAl alloy model

Interatomic potentials have great importance in the analysis and calculations of some parameters in atomic scale. These calculations are realised by the computer simulation techniques. In the present study, a molecular dynamics (MD) simulation method which allows the system to vary in shape and size was used for the investigation of diffusionless phase transformation in Ni–37.5 at.%Al alloy model which exhibits shape memory effect in this composition. Interatomic forces were determined by the gradient of Lennard-Jones potential function, and the potential parameters were optimised by the MD simulations. Optimisation was done corresponding to the crystal lattice properties and melting point. The crystallographic properties of the alloy were investigated in high temperature phase (B2-type super-lattice) field, and diffusionless phase transformation was carried out by means of a rapid cooling method. Also, lattice faults were observed in the crystal structure after the transformation.     

Study of Friction between Liquid Crystals and Crystalline Surfaces by Molecular Dynamic Simulations

The lubrication characteristics of liquid crystal (LC) molecules sheared between two crystalline surfaces obtained from molecular dynamics (MD) simulations are reported in this article. We consider a coarse-grained rigid bead-necklace model of the LC molecules confined between two atomic surfaces subject to different shearing velocities. A systematic study shows that the slip length of LC lubrication changes significantly as a function of the LC-surface interaction energy, which can be well described though a theoretical curve. The slip length increases as shear rate increases at high LC-surface interaction energy. However, this trend can not be observed for low interaction energy. The orientation of the LC molecules near the surface is found to be guided by the atomics surfaces. The influence of temperature on the lubrication characteristics is also discussed in this article.     

水高温高密度状态方程理论研究

 采用MCR方法计算由exp-6势描述的水分子作用体系的pVT状态方程。与MD数值模拟结果比较后发现，由于水分子间强烈的吸引作用有利于分子有序化过程的发生，在较高温度（2 000~4 000 K）条件下，水分子作用体系仍呈现出固态特征。该物相区内体系的热力学性质不能用MCR理论描述但MCR理论准确预言了水分子作用体系高温液相区pVT状态方程。     

Finite-temperature quasicontinuum: molecular dynamics without all the atoms

Using a combination of statistical mechanics and finite-element interpolation, we develop a coarse-grained (CG) alternative to molecular dynamics (MD) for crystalline solids at constant temperature. The new approach is significantly more efficient than MD and generalizes earlier work on the quasicontinuum method. The method is validated by recovering equilibrium properties of single crystal Ni as a function of temperature. CG dynamical simulations of nano-indentation reveal a strong dependence on temperature of the critical stress to nucleate dislocations under the indenter.     

Determination of ion quantity by using low-temperature ion density theory and molecular dynamics simulation

In this paper, we report a method by which the ion quantity is estimated rapidly with an accuracy of 4%. This finding is based on the low-temperature ion density theory and combined with the ion crystal size obtained from experiment with the precision of a micrometer. The method is objective, straightforward, and independent of the molecular dynamics(MD)simulation. The result can be used as the reference for the MD simulation, and the method can improve the reliability and precision of MD simulation. This method is very helpful for intensively studying ion crystal, such as phase transition,spatial configuration, temporal evolution, dynamic character, cooling efficiency, and the temperature limit of the ions.     

Anomalous scattering in supercooled ST2 water

ABSTRACT

Large-scale molecular dynamics (MD) simulations of systems containing up to 256,000 molecules were performed to investigate the scattering behaviour of the ST2 water model at deeply supercooled conditions. The simulations reveal that ST2 exhibits anomalous scattering, reminiscent of that observed in experiment, which is characterised by an increase in the static structure factor at low wavenumbers. This unusual behaviour in ST2 is linked with coupled fluctuations in density and local tetrahedral order in the liquid. The Ornstein–Zernike correlation length estimated from the anomalous scattering component exhibits power-law growth upon cooling, consistent with the existence of a liquid–liquid critical point (LLCP) in the ST2 model at ca. 245 K. Further, spontaneous liquid–liquid phase separation is observed upon thermally quenching a large system with 256,000 water molecules below the predicted critical temperature into the two-phase region. The large-scale MD simulations therefore confirm the existence of a metastable liquid–liquid phase transition in ST2 and support findings from previous computational studies performed using smaller systems containing only a few hundred molecules. We anticipate that our analysis may prove useful in interpreting recent scattering experiments that have been performed to search for an LLCP in deeply supercooled water.     

Pressure dependence of crystal structure and molecular packing in anthracene

Anthracene molecular crystal has been investigated up to a pressure of 10.5 GPa at room temperature using variable shape variable size Monte Carlo simulations in an isothermal–isobaric ensemble. We have reported various structural quantities, such as cell parameters and unit cell volume, as a function of pressure and compared them with the experimental results [J. Chem. Phys. 119, 1078 (2003)]. The pressure dependence of angles θ, δ and χ which describe the relative packing of molecules in the crystal has been calculated. We report that anthracene molecular crystal does not exhibit any first order phase transition up to a pressure of 10.5 GPa which is consistent with the experimental observations by Oehzelt et al. [Phys. Rev. B 66, 174104 (2002)]. The calculated equation of state (EOS) has been fitted to a Murnaghan-type EOS with good agreement. The calculated bulk modulus and the pressure derivative of bulk modulus are 8.2 GPa and 8.9 respectively which are in agreement with the experimentally calculated values.     

Grain boundary premelting of monolayer ices in 2D nano-channels

ABSTRACT

We employ molecular-dynamics (MD) simulations to study grain boundary (GB) premelting in ices confined in two-dimensional hydrophobic nano-channels. Premelting transitions are observed in symmetric tilt GBs in monolayer ices and involve the formation of a premelting band of liquid phase water with a width that grows logarithmically as the melting temperature is approached from below, consistent with the existing theory of GB premelting. The premelted GB is found rough for a broad range of temperature below the melting temperature, the two ice-premelt interfaces bounding the melted GB are engaged with long wave-length parallel coupled fluctuations. Based on current MD simulation study, one may expect GB premelting transitions exist over a wide range of low dimensional phases of confined ice and shows important consequences for crystal growth of low dimensional ices.     

金红石型氧化物晶体结构与性能的分子动力学研究

采用分子动力学的方法,利用新的势能模型,对金红石型氧化物TiO2,GeO2和SnO2完整晶体的热性能和随压力变化特性进行计算模拟;在完整晶体中,引入肖特基型点缺陷,以研究和比较两种状态下的差别,井对GeO2-SnO2固溶体的高温固溶状态进行计算模拟。     

Phase behaviour and interfacial properties of ternary system CO2 + n-butane + n-decane: coarse-grained theoretical modelling and molecular simulations

ABSTRACT

This work illustrates the application of a three-party approach based on theoretical modelling, molecular dynamic (MD) simulations and available experimental data for describing the phase equilibrium and interfacial properties for the ternary system: carbon dioxide + n-butane + n-decane and its corresponding binary sub-systems at 344.3 K. Specifically, a coarse-grained force field is employed for both theoretical predictions and MD. The interfacial region is described by the square gradient theory where the homogenous Helmholtz energy density contribution is provided by the Statistical Associated Fluid Theory equation of state for potentials of variable range for molecules conformed of segments interacting through the Mie potential (SAFT-VR Mie) and MD simulations in the canonical ensemble where the molecules are represented by a coarse-grained Mie force field. The novelty here is that both the theory and the simulations uniquely share the same underlying intermolecular potentials; hence, the experimental data are employed to verify both the theory and simulations. In this schema, the ternary mixture is full predictive as its parameters are only based on pure fluids parameters and binary interactions. It is observed that the phase equilibria and the interfacial properties are equally well represented by the used approach.     

V5+掺杂TiO2金红石结构的分子动力学

采用经典离子晶体相互作用势模型， 进行量子化修正，对TiO2金红石相替位式掺杂0.5mol％V5+进行分子动力学模拟，计算了常温常压下的键长、键角的分布函数，分析了晶体的微结构变化以及V5+对TiO6八面体的影响．模拟结果表明，掺杂V5+会导致Ti-O、O-O键长以及O-Ti-O键角的分布范围变宽，其中，键长的主峰位置未发生显著的移动， 而键角主峰向低角度方向移动． 由于V5+比Ti4+有较高的正电荷和较小的离子半径， 因此V5+向氧八面体间空隙扩散，且掺杂V5+迁移出原来的O6八面体，从而导致空隙邻近的TiO6八面体发生严重扭曲．在低掺杂情况下(0.5 mol％)虽然掺杂易造成结构畸变，但模拟体系整体仍维持金红石结构；所得模拟的键长变化、掺杂V5+迁移、晶相等结构特性与FTIR、Raman、XRD、ESR等实验结果相一致．     

Evaluating the effects of loading parameters on single-crystal slip in tantalum using molecular mechanics

This study is aimed at developing a physics-based crystal plasticity finite element model for body-centred cubic (BCC) metals, through the introduction of atomic-level deformation information from molecular dynamics (MD) investigations of dislocation motion at the onset of plastic flow. In this study, three critical variables governing crystal plasticity mediated by dislocation motion are considered. MD simulations are first performed across a range of finite temperatures up to 600K to quantify the temperature dependence of critical stress required for slip initiation. An important feature of slip in BCC metals is that it is not solely dependent on the Schmid law measure of resolved shear stress, commonly employed in crystal plasticity models. The configuration of a screw dislocation and its subsequent motion is studied under different load orientations to quantify these non-Schmid effects. Finally, the influence of strain rates on thermal activation is studied by inducing higher stresses during activation at higher applied strain rates. Functional dependence of the critical resolved shear stress on temperature, loading orientation and strain rate is determined from the MD simulation results. The functional forms are derived from the thermal activation mechanisms that govern the plastic behaviour and quantification of relevant deformation variables. The resulting physics-based rate-dependent crystal plasticity model is implemented in a crystal plasticity finite element code. Uniaxial simulations reveal orientation-dependent tension–compression asymmetry of yield that more accurately represents single-crystal experimental results than standard models.     

Distribution and thermophysical properties of n-heptacosane molecules confined in carbon nanotube

Molecular dynamics (MD) simulations were performed to provide an insight about the molecule distribution and thermophysical properties of n-heptacosane confined in the (25, 25) single-walled carbon nanotube (CNT). The results show that an orderly distribution of n-heptacosane molecules along the CNT inner wall is clearly observed. Meanwhile, n-heptacosane confined in CNT exhibits an increased self-diffusion coefficient, a decreased melting point and an enhanced thermal conductivity, compared to the bulk. The simulations reveal that MD is an effective and convenient method to understand the variation characteristics of alkane-based phase change materials confined in CNT on molecular and atomic scale.     

How to identify dislocations in molecular dynamics simulations?

Dislocations are of great importance in revealing the underlying mechanisms of deformed solid crystals.With the development of computational facilities and technologies,the observations of dislocations at atomic level through numerical simulations are permitted.Molecular dynamics(MD)simulation suggests itself as a powerful tool for understanding and visualizing the creation of dislocations as well as the evolution of crystal defects.However,the numerical results from the large-scale MD simulations are not very illuminating by themselves and there exist various techniques for analyzing dislocations and the deformed crystal structures.Thus,it is a big challenge for the beginners in this community to choose a proper method to start their investigations.In this review,we summarized and discussed up to twelve existing structure characterization methods in MD simulations of deformed crystal solids.A comprehensive comparison was made between the advantages and disadvantages of these typical techniques.We also examined some of the recent advances in the dynamics of dislocations related to the hydraulic fracturing.It was found that the dislocation emission has a significant effect on the propagation and bifurcation of the crack tip in the hydraulic fracturing.     

An application of cell theory to molecular models of n-alkane solids

Solid phase properties for hard sphere chain molecular models of n-alkanes are calculated using the cell theory, and a numerical method for implementation of cell theory for chain molecules is described. Good agreement with Monte Carlo simulations for solid phase properties is obtained from the theory. By using cell theory for the solid phase and an equation of state for the fluid phase, solid-phase equilibrium can be calculated. The predictions are in quite good agreement with Monte Carlo simulation results. Cell theory is used to assess the impact of an approximate treatment used in earlier work for the effect of the temperature dependence of the molecular flexibility upon the solid phase properties of a hard chain model with a realistic torsional potential.     

High-resolution FTIR study of the para-terphenyl phase transition at high pressure

High-resolution infrared (IR) spectroscopy has been used to investigate the pressure-induced (0-11 kbar) polymorphic phase transition of crystalline para-terphenyl at low temperature (25 K). A number of doublet bands observed in low-pressure triclinic p-terphenyl were observed to coalesce in the high-pressure monoclinic phase. The coalescing of doublet bands was attributed to changes in factor group (Davydov) splittings associated with the transition from a low-pressure triclinic phase to a high-pressure monoclinic phase. The bands that ‘disappear’ also do not correlate with frequency changes associated with changes in molecular symmetry. Molecular dynamics (MD) simulations at low temperature (20 K) yield a non-planar average molecular structure for the high-pressure monoclinic phase, in contrast to the high-temperature monoclinic phase. The MD simulations also reveal a broadening of the distribution of ring torsion angles near the triclinic-monoclinic phase transition pressure.     

Methods for calculating the desorption rate of an isolated molecule from a surface: Water on MgO(0 0 1)

We present two types of Molecular Dynamics (MD) simulation for calculating the desorption rate of molecules from a surface. In the first, the molecules move freely between two surfaces, and the desorption rate is obtained either by counting the number of desorption events in a given time, or by looking at the average density of the molecules as a function of distance from the surface and then applying transition state theory (TST). In the second, the potential of mean force (PMF) for a molecule is determined as a function of distance from the surface and the desorption rate is obtained by means of TST. The methods are applied to water on the MgO(0 0 1) surface at low coverage. Classical potentials are used so that long simulations can be performed, to minimise statistical errors. The two sets of MD simulations agree well at high temperatures. The PMF method reproduces the 0 K adsorption energy of the molecule to within 5 meV, and finds that the well depth of the PMF is not linear with temperature. This implies the prefactor frequency f in the Polanyi-Wigner equation is a function of temperature, increasing at lower temperatures due to the reduction of the available configuration space associated with an adsorbed molecule compared with a free molecule.     

Thermal conductivity and diffusion behaviour of lauric acid confined in carbon nanotubes as heat storage materials

The thermal conductivity and diffusion behaviour of lauric acid (LA) confined in single-walled carbon nanotube (CNT) with the filling ratio of 80% are investigated by molecular dynamics (MD) simulations. It is found that the concentric multilayer LA tubes are clearly observed under the interaction of CNT and LA and the hydrogen bonds (HB) among LA molecules in a confined environment. Due to the phonon scattering in low-frequency and the high-thermal conductivity characteristics of CNT, the axial thermal conductivity of CNT/LA is 49–57% lower than that of empty CNT and 115–188 times higher than that of crystal LA at the temperature range of 280 and 360?K. The confined LA molecules move as a whole cluster due to the long-lasting HB action and travel much faster than the bulk.