Self-consistent Shaw optimized model potential: Application to the determination of structural and atomic transport properties of liquid alkali metals by molecular dynamics simulations

The ‘first-principles’ fully non-local and energy-dependent optimized model potential (OMP) derived by Shaw is developed further. In contrast to Shaw's original paper, OMP parameters are derived in a self-consistent manner that does not rely on knowledge of experimental values of the ionization and cohesive energies. To our knowledge, this is the first time that this method has been used for effective potential calculations. In an application to liquid Li, Na, and K alkali metals, we used OMP pseudopotential-based interactions between ions to carry out standard molecular dynamics simulations. In the calculations, the ionic structure for the liquid state was first checked at a temperature near the melting point. Similar accurate calculations, but for atomic transport properties, predict the temperature dependence of the self-diffusion coefficients. The theoretical results obtained are in overall agreement with available experimental measurements. Thus, one can have some confidence in the ability of the optimized model potential to give a good representation of the physical properties of these alkali ions in the liquid environment.     

液态纯铁自扩散系数研究

液态金属固有的良好导热性能和很好的流动性能,逐渐受到了工程领域越来越多的重视。但由于液相扩散系数的测量困难,目前未见液态纯铁的自扩散系数报道。本文以液态纯铁正则系综为研究对象,采用分子动力学模拟方法对其自扩散系数进行了模拟分析,研究结果表明液态纯铁自扩散系数随温度增加呈现出了较大的变化,应用自扩散系数与温度存在的Arrhenius关系拟合出了液态纯铁的自扩散系数经验公式,进一步获得了液态纯铁原子的平均自由程计算关系式。     

从常温常压到超临界乙醇的分子动力学模拟

采用分子动力学方法系统地研究了从常温常压到超临界状态乙醇的热力学性质、结构性质和动力学性质.模拟发现随着温度的升高,体系焓值增大,乙醇分子间的氢键作用减弱,自扩散系数增大;随着压强的增大,乙醇分子间的氢键作用增强,自扩散系数减小;乙醇自扩散系数在液相区随温度变化不明显,在气相区随压强增大很快减小,超临界区乙醇的自扩散系数比液相区大十几倍.温度和压强对乙醇自扩散系数的影响可通过密度来体现.与常温常压相比,超临界条件下的乙醇体系因密度涨落存在分子聚集现象,且在低密度区域更显著;乙醇分子间的氢键作用明显减弱,结     

Atomic structure and diffusivity in liquid Al80Ni20 by ab initio molecular dynamics simulations

The atomic structure and diffusivity in liquid Al₈₀Ni₂₀ are studied by ab initio molecular dynamics simulations. The local structures are analyzed by the pair correlation function, structure factor, coordinate number, Honneycutt–Anderson bond pair, and Voronoi tessellation methods. It is observed that the amount of icosahedral clusters increases, and the liquid becomes more ordered as the temperature decreases. The predicted self-diffusion coefficients of Al and Ni via the mean square displacements are very close to each other and agree well with the quasi-elastic neutron scattering measurements in the literature. The observation of equal self-diffusivity of Al and Ni is attributed to the formation of local solute-centered polyhedra, coupling the migration of Al and Ni. The Manning dynamic correlation factor is evaluated and found to be close to unity. The predicted interdiffusion coefficients using the Darken equation agree well with experimental data in the literature.     

吸附原子在Ag,Pt,Au(110)表面上的自扩散现象

采用表面嵌入势，用静态计算和分子动力学方法研究了Ag,Pt,Au单个吸附原子在(110)表面上的自扩散现象.分别给出了跳跃机制和交换机制所对应的能量变化曲线及相关原子的运动轨迹，分析了这三种不同金属(110)表面上的自扩散特点，结果与分子动力学模拟及有关的实验结果相符合.     

Study of molecular mobility of fluid in zeolite NaX.

The self-diffusion of n-decane in zeolite NaX was studied by NMR PFG method. The situation when the liquid was only in the crystalline channels was studied in details. The restricted molecular motion of liquid in the crystalline channels was observed. The reasons of the anomalous self-diffusion of n-decane in zeolite bed were stated. The technique of the determination of the genuine self-diffusion coefficient in such porous systems was proposed. The genuine self-diffusion coefficients for system NaX/n-decane were obtained.     

Molecular dynamics of liquid alkaline-earth metals near the melting point

Results of the studies of the properties like binding energy, the pair distribution function g(r), the structure factor S(q), specific heat at constant volume, velocity autocorrelation function (VACF), radial distribution function, self-diffusion coefficient and coordination number of alkaline-earth metals (Be, Mg, Ca, Sr and Ba) near melting point using molecular dynamics (MD) simulation technique using a pseudopotential proposed by us are presented in this article. Good agreement with the experiment is achieved for the binding energy, pair distribution function and structure factor, and these results compare favourably with the results obtained by other such calculations, showing the transferability of the pseudopotential used from solid to liquid environment in the case of alkaline-earth metals.     

Universal Scaling Law for Atomic Diffusion and Viscosity in Liquid Metals

The recently proposed scaling law relating the diffusion coefficient and the excess entropy of liquid [Samanta A et al. 2004 Phys. Reu. Lett. 92 145901; Dzugutov M 1996 Nature 381 137], and a quasi-universal relationship between the transport coefficients and excess entropy of dense fluids [Rosenfeld Y 1977 Phys. Rev. A 15 2545],are tested for diverse liquid metals using molecular dynamics simulations. Interatomic potentials derived from the glue potential and second-moment approximation of tight-binding scheme are used to study liquid metals.Our simulation results give sound support to the above-mentioned universal scaling laws. Following Dzugutov,we have also reached a new universal scaling relationship between the viscosity coefficient and excess entropy.The simulation results suggest that the reduced transport coefficients can be expressed approximately in terms of the corresponding packing density.     

Diffusion coefficients of vacancies and interstitials along tilt grain boundaries in molybdenum

The diffusion coefficients of vacancies and interstitials along symmetrical tilt grain boundaries in molybdenum have been calculated using the molecular dynamics method. The migration energies of defects have been obtained. The activation energy and coefficients of grain boundary self-diffusion have been deter-mined. A comparison of the obtained results with the studies of other authors indicates that boundaries formed between particles in the powder in sintering experiments have a higher diffusion activity as compared to stable grain boundaries in polycrystals.     

Quantum molecular dynamics simulation of hydrogen diffusion in zirconium hydride

The behavior of hydrogen in zirconium hydride in the high-temperature range has been investigated using the quantum molecular dynamics method. The δ phases of compositions ZrH_1.75 and ZrH₂ and the liquid phase are considered. The self-diffusion coefficients of hydrogen are calculated as a function of the temperature in the range from 1000 to 6000 K. For the ZrH_1.75 and ZrH₂ hydrides, the obtained values are close to each other. At temperatures of 1000–2000 K, the hydrogen diffusion is determined not only by the mobility of hydrogen atoms but also by the transition from the energetically favorable tetrahedral positions into the excited state. The obtained values of the diffusion coefficients in the temperature range of 1000–1200 K are in good agreement with the experimental data.     

Molecular dynamics studies of an m-6-8 fluid

The pressure, energy and self-diffusion coefficient have been calculated for an 11-6-8 fluid using the method of molecular dynamics. A comparison with data for argon, krypton and xenon is presented. It is shown that agreement between theory and experiment for the thermodynamic properties is generally within the estimated precision of the calculated and experimental values, provided three-body and quantum corrections are included in the calculation, except when the density approaches the triple-point density. Agreement between theoretical and experimental self-diffusion coefficients is satisfactory at all densities after allowance is made for the long-time behaviour of the velocity autocorrelation function. We demonstrate that three-body forces are important in the liquid and that our estimates of such forces are very close to those suggested by Barker, Fisher and Watts.     

Contributions of different mechanisms of self-diffusion in face-centered cubic metals under equilibrium conditions

The contributions of different mechanisms of self-diffusion in face-centered cubic metals Ni, Cu, and Al at thermodynamic equilibrium have been analyzed using the molecular dynamics method. The vacancy, divacancy, and cyclic mechanisms of self-diffusion, as well as the mechanisms involving the vacancy migration to the second coordination sphere and the formation and recombination of Frenkel pairs, have been considered. It has been shown that the second in contribution to the self-diffusion after the vacancy mechanism in the metals under consideration is the migration of divacancies. The third is the mechanism involving the formation and recombination of dynamic Frenkel pairs. The cyclic mechanisms (with simultaneous atomic displacements) and the vacancy migration directly to the second coordination sphere in face-centered cubic metals are unlikely.     

Deuterium stimulated-echo-type PGSE NMR experiments for measuring diffusion: application to a liquid crystal.

The accessibility of molecular self-diffusion coefficients in anisotropic materials, such as liquid crystals or solids, by stimulated-echo-type (2)H PGSE NMR is examined. The amplitude and phase modulation of the signal in the stimulated-echo-type sequence by the static quadrupole coupling during the encoding/decoding delays is suppressed by adjusting the pulse flip angles and the phase cycle. For nuclei that experience both nonnegligible quadrupole and dipole couplings, the application of magic echoes during the evolution periods of stimulated echo is demonstrated as a helpful technique in the case of slow diffusion. These findings are demonstrated by experimental results in the thermotropic liquid crystal of partially deuterated 8CB. The obtained diffusion coefficients are also compared to data obtained by a (1)H homonuclear-decoupling-type PGSE NMR method in the same material.     

Mass and thermal transport in liquid Cu-Ag alloys

In this paper, the diffusion, thermodynamic and thermotransport properties in Cu–Ag liquid alloys are extensively investigated with molecular dynamics over a wide composition and temperature range. The simulations are performed with the most reliable EAM potential. The Green-Kubo formalism is employed for calculating transport properties. It is found that the reduced heat of transport in Cu–Ag is very small (about 0.10?eV in absolute value) and almost temperature independent. Further it is found that the interdiffusion coefficient together with both self-diffusion coefficients are almost composition independent. In Cu–Ag, the thermodynamic factor is found to be less than unity whereas the Manning factor is greater than unity (with significant composition and temperature dependence) and their product is very close to 1.     

Transitioning model potentials to real systems

The parameters of two pair potentials that describe argon over its entire liquid phase at a fixed pressure were optimized through a novel application of constant temperature and pressure molecular dynamics (NPT-MD) and Monte Carlo (NPT-MC) computer simulations. The forms of these potentials were those of a modified Lennard-Jones potential and a Lennard-Jones potential. The optimized potential determined using NPT-MD simulations reproduces experimental densities, internal energies and enthalpies with an error less than 1% over most of the liquid range and yields self-diffusion coefficients that are in excellent agreement with experiment. The results using the potential determined by NPT-MC simulations are in almost as good agreement with deviations from experiment of no more than 5.89% for temperatures up to vaporization. Additionally, molar volumes predicted using this potential at pressures in the range 100–600 atm and over temperatures in the range 100–140K were within 0.83% of experimental values. These results show that, when properly parametrized, Lennard-Jones-like potentials can describe a system well over a large temperature range. Further, the method introduced is easy to implement and is independent of the form of the interaction potential used.     

Computer simulation of the structure of liquid cesium and determination of the pair potentials over a wide temperature range

Using diffraction data for liquid cesium structure over a wide temperature range, models of liquid cesium are constructed and the effective pair potentials are extracted using the theory of liquids. The iterative procedure proposed by L. Reatto is used. In the range 323-1923 K the pair potentials are weakly temperature-dependent. The potentials extracted from the diffraction data differ from the potentials calculated using the Animalu-Heine pseudopotential. The self-diffusion coefficients in liquid cesium are determined. Their temperature dependence is described satisfactorily by a power-law function. Zh. éksp. Teor. Fiz. 115, 50–60 (January 1999)     

液态过渡金属Pd和Pt的结构与微观动力学行为

采用反映原子多体相互作用的镶嵌原子模型和有效偶势模型及分子动力学计算机模拟技术,研究了液态过渡金属Pd和Pt在熔点附近的结构与微观动力学行为。计算了Pd和Pt分别在1818和2037K时的双体分布函数、平均平方位移、自扩散系数和速度自相关函数,讨论了多体相互作用对上述各物理量的影响。结果表明,多体相互作用对液态Pd和Pt的局域结构影响较小,但对反映微观动力学行为的平均平方位移、自扩散系数和速度自相关函数的影响较大。 关键词：     

Molecular dynamics characterization of icosahedral short range order in undercooled copper

The stability of undercooled simple metals is still an intriguing problem for materials science and technology. There is not consensus on the role played by the icosahedral short range order during undercooling. The scenario is even less clear for undercooled metals under external pressure. Extensive molecular dynamics simulations, based on an empirical tight-binding interatomic potential, are performed to explain experimental results recently obtained on liquid and undercooled liquid copper. A common neighbour analysis is used to fully characterize the icosahedral short range order in both undercooled and liquid systems. Moreover, the effect of pressure on icosahedral short range order, is addressed and rationalized. External pressure increases the probability to find atomic bonds with icosahedral symmetry both in the liquid and in the undercooled copper.     

Theoretical studies of adatom diffusion on metal surfaces

We propose in this paper a theoretical model to investigate surface self-diffusion of single adatoms on two different low-index planes, closely packed (001) and densely packed (111), of face-centered-cubic rhodium, nickel and copper metal crystals. Two realistic model potentials are applied to describe the interatomic interaction of the adatom-substrate systems. The first model is a Morse-type potential, which involves several empirical fittings of bulk of solid. The second, newly popular, potential was introduced by Sutton and Che, which incorporates many-body effects. With these potentials, conventional molecular dynamics (MD) is employed to obtain trajectories of the atoms. The average squared didplacements are computed for a range of initial kinetic energies, and the surface diffusion constants can be obtained by means of the Einstein relation. The estimated random walk exponential prefactors and activation energies exhibit an Arrhenius behavior, and are compared with previous results.