Angular Dependent Potential Research Articles

Angular-dependent interatomic potential for the Cu–Ta system and its application to structural stability of nano-crystalline alloys

Abstract Atomistic computer simulations are capable of providing insights into physical mechanisms responsible for the extraordinary structural stability and strength of immiscible Cu–Ta alloys. To enable reliable simulations of these alloys, we have developed an angular-dependent potential (ADP) for the Cu–Ta system by fitting to a large database of first-principles and experimental data. This, in turn, required the development of a new ADP potential for elemental Ta, which accurately reproduces a wide range of properties of Ta and is transferable to severely deformed states and diverse atomic environments. The new Cu–Ta potential is applied for studying the kinetics of grain growth in nano-crystalline Cu–Ta alloys with different chemical compositions. Ta atoms form nanometer-scale clusters preferentially located at grain boundaries (GBs) and triple junctions. These clusters pin some of the GBs in place and cause a drastic decrease in grain growth by the Zener pinning mechanism. The results of the simulations are well consistent with experimental observations and suggest possible mechanisms of the stabilization effect of Ta.

Angular dependent potential for α-boron and large-scale molecular dynamics simulations

Both quantum mechanical and molecular-dynamics (MD) simulations of α-boron are done at this work. Angular dependent interatomic potential (ADP) for boron is obtained using force-matching technique. Fitting data are based on ab initio results within −20..100 GPa pressure range and temperatures up to 2000 K. Characteristics of α-boron, obtained using ADP potential such as bond lengths at equilibrium condition, bulk modulus, pressure-volume relations, Gruneisen coefficient, thermal expansion coefficient are in good agreement with both ab initio data, obtained in this work and known experimental data. As an example of application, the propagation of shock waves through a single crystal of α-boron is also explored by large-scale MD simulations.

Orientation dependences of atomic structures in chemically heterogeneous Cu50Ta50/Ta glass-crystal interfaces

Molecular dynamics simulations based on an angular-dependent potential were performed to examine the structural properties of chemically heterogeneous interfaces between amorphous Cu50Ta50 and crystalline Ta. Several phenomena, namely, layering, crystallization, intermixing, and composition segregation, were observed in the Cu50Ta50 region adjacent to the Ta layers. These interfacial behaviors are found to depend on the orientation of the underlying Ta substrate: Layering induced by Ta(110) extends the farthest into Cu50Ta50, crystallization in the Cu50Ta50 region is most significant for interface against Ta(100), while inter-diffusion is most pronounced for Ta(111). It turns out that the induced layering behavior is dominated by the interlayer distances of the underlying Ta layers, while the degree of inter-diffusion is governed by the openness of the Ta crystalline layers. In addition, composition segregations are observed in all interface models, corresponding to the immiscible nature of the Cu-Ta system. Furthermore, Voronoi polyhedra ⟨0,5,2,6⟩ and ⟨0,4,4,6⟩ are found to be abundant in the vicinity of the interfaces for all models, whose presence is believed to facilitate the structural transition between amorphous and body centered cubic.

States of an on-axis two-hydrogenic-impurity complex in concentric double quantum rings

Abstract The energy structure of an on-axis two-donor system ( D 2 0 ) confined in GaAs concentric double quantum rings under the presence of magnetic field and hydrostatic pressure was analyzed. Based on structural data for the double quantum ring morphology, a rigorous adiabatic procedure was implemented to separate the electrons' rapid in-plane motions from the slow rotational ones. A one-dimensional equation with an effective angular-dependent potential, which describes the two-electron rotations around the common symmetry axis of quantum rings was obtained. It was shown that D 2 0 complex characteristic features are strongly dependent on the quantum ring geometrical parameters. Besides, by changing the hydrostatic pressure and magnetic field strengths, it is possible to tune the D 2 0 energy structure. Our results are comparable to those previously reported for a single and negative ionized donor in a spherical quantum dot after a selective setting of the geometrical parameters of the structure.

Theoretical Study of the Lithium Diffusion in the Crystalline and Amorphous Silicon as well as on its Surface

Using the PAW DFT-GGA method and numerical solving of master equation the diffusion rates of lithium atoms inside both crystal and amorphous silicon of LixSi (x= 0..0.5) composition have been calculated for different temperatures. It is shown the diffusion rate for amorphous silicon is ~10 times greater than that for the crystal silicon. For both structures the rate is increased by 1.5-2 orders of magnitude while the lithium concentration is increased up to 0.5 value. This should result in that the LixSi/Si interface will be sharp. This fact has been further confirmed using molecular dynamic calculations based on Angular Dependent Potential (ADP) model. Also binding energies of Li atoms lying on different sites of Si (001) surface as well as the potential barriers for the atom jumps both along the surface and in the subsurface layers have been calculated. The data show the Li atoms move along the surface very easily but their jumps into subsurface layers are very difficult due to the high potential barrier values.

Structural and dynamical properties of heterogeneous solid–liquid Ta–Cu interfaces: A molecular dynamics study

Molecular dynamics simulations based on an angular-dependent potential were performed to examine the structural and dynamical properties of chemically heterogeneous solid–liquid interfaces formed between Ta and Cu. Layering is found in liquid Cu adjacent to solid Ta, and its behavior depends on the orientation of the underlying Ta substrate. In the Ta (110)/Cu interface, layering in Cu extends more than 15Å away from the interface, and no intermixing or prefreezing is observed. While in the (100) interface, besides significant layering in Cu, intermixing of Cu and Ta is also seen in the vicinity of the interface. Furthermore, the first layer of Cu is prefrozen, forming a 1×1 structure relatively to the underlying Ta; while atoms in other Cu layers are rather mobile. In the (111) interface, the layering is very weak, extends no more than 6Å into the liquid, the interfacial alloying is however rather significant. The orientation dependence in interfacial structure in turn leads to orientation dependence in interfacial energy, interfacial width, and diffusivity of atoms adjacent to the interface. Specially, the diffusion coefficients of Cu depend not only on the orientation of the solid substrate, but also on their relative motion with respective to the interface: the in-plane component is greater than the out-of-plane one, which is also found to have a structural origin.

Excess entropy scaling for transport coefficients: diffusion and viscosity in liquid metals

A semi-empirical universal relationship proposed by Rosenfeld for dimensionless transport coefficients in dense fluids, and a universal scaling law proposed by Dzugutov for atomic diffusion in liquids, 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; the angular dependent Tersoff potential and Stillinger–Weber potential are used to investigate Si. Our simulation results give sound support to the above-mentioned universal scaling laws. Following Dzugutov, we have also arrived at a universal scaling relationship between the viscosity coefficient and excess entropy. The simulation results suggest that there approximately exists an exponent correlation between the reduced transport coefficients and the corresponding packing density.

ASSESSING THE EFFECT OF COULOMB REPULSION ASYMMETRY ON ELECTRON PAIRING

Coulomb repulsion between two moving electrons loses its spherical symmetry due to relativistic effects. In presence of a uniform positive ion background, this asymmetry uncovers an angular dependent attraction potential in the direction of motion. The quantum mechanical response to such an attraction potential is obtained through perturbation. It is shown that the transition amplitude between states with the symmetry of the attraction potential becomes negative and if the density of states is anisotropic, occurrence of a superconducting state becomes possible.

Ab Initio Study of the Ne(1S)−CN(2Σ+) van der Waals Complex

The Ne(1S)−CN(2Σ+) van der Waals complex, described by Jacobi coordinates (R, ϑ), was studied using the supermolecular and perturbation UHF as well as RHF ab initio treatments with the inclusion of correlation energy. The computed potential energy surface at the restricted MP2 level of the theory reveals a single minimum, located near the T-shaped configuration at ϑ = 110°, R = 3.7 Å, and its well depth amounts to −145.9 μEh. When compared with the RMP2 potential, the UMP2 surface is more repulsive in the region of the minimum and the double-well character of the angular-dependent potential curves appears about R ≈ 3.7 Å. The position of the minimum is nearer to the C end, at R ≈ 3.7 Å and ϑ = 50°. The influence of the selected geometry variations on the fundamental energy components, obtained from intermolecular perturbation theory, was also investigated. The separation of the interaction energy shows that the shape and location of these minima are primarily determined by the anisotropy of the exchange-penetration and dispersion components.

High-pressure Raman spectra of the acetone carbonyl stretch in acetone-methanol mixtures

The isotropic and anisotropic Raman bands of the carbonyl mode in acetone do not coincide, a phenomenon caused by resonant transfer of vibrational energy coupled with an angular dependent intermolecular potential. We report on the frequency shifts, line width changes, and noncoincidence of these bands in mixtures with methanol as the density, temperature, and acetone mole fraction (x A ) are independently varied. Increased density red shifts both signals; it also narrows the anisotropic band while not greatly altering the isotropic band. Increased temperature blue shifts both bands and broadens them (considerably, for the anisotropic mode)

Adsorption of polar molecules at a wall: Monte Carlo simulations and integral equations

In this paper we investigate the influence of an angular-dependent adsorption potential on the structure of a dipolar hard sphere fluid in the vicinity of a hard wall. We report Monte Carlo (MC) simulation results for the density profile, the orientational structure, the polarization and the electrical potential profiles and make detailed comparisons with the theoretical results obtained from integral equations theories. We propose a new version of the reference linearized hypernetted chain approximation (RLHNC) which clears up its relation with the mean spherical approximation (MSA). Good agreement with MC results is reached when one takes as a reference system the dipolar fluid in the absence of adsorption potential.

The equivalent potential for relativistic confined systems

In previous papers a method of obtaining bound states and wavefunctions for confined relativistic systems was presented. The input is the asymptotic expansion of the two-point functions. Confinement is imposed by systematic removal of the two-particle cut. We extend this method by developing an equivalent (angular momentum dependent) potential, which gives the correct wavefunction to a given order of R, the infrared scale parameter. We prove the uniqueness of the wavefunction by requiring that there are no CDD poles and by the connection of our moment conditions to the requirement that the residues of the bound-state poles must be positive. Finally we test the bound-state approximation for a system defined by an equivalent potential V(r) = λ 2 tanh 2( g 2r λ ) . Although in this case there is a threshold we still find excellent results when λ 2 g 2 is large, i.c., when there are many bound states.

The angular dependent potential for the H 2—inert gas systems from total collision cross section measurements with state selected molecules

Results of total collision cross section measurements with orientationally selected molecules are presented, for the systems H 2H 2, He, Ne, Ar, Kr and Xe. Anisotropy parameters for a Lennard-Jones ( m,6) type potential are given.

Measurements of the influence of the angular dependent intermolecular potential on the total collision cross section for molecular hydrogen and various scattering partners

For H 2 molecules colliding with Ar, Kr, Xe, CO 2, N 2 and SF 6 the total collision cross section is measured, applying state selection to the H 2 molecule. The preferential orientation of the H 2 molecule in the J = 1, m J = 0 state is used to determine the influence of the angular dependent part of the intermolecular potential.

Modified free energy averaged potential for complex polar fluids

Free energy averaged potential for low density simple polar fluids used by Danon et al. has been modified for complex polar fluids like water and ammonia by introducing a dipole-quadrupole interaction term in Stockmayer angular dependent potential. The resulting temperature dependent effective pair potential is used for calculation of second virial coefficient and specific heat of water vapor at 1 and 10 atm pressure. Comparison of results shows an improvement of new potential over DA potential.

Initial Pressure Dependence of the Viscosity of Polar Gases

The initial pressure dependence of the viscosity of polar gases is studied within the approximate treatment of Stogryn and Herschfelder. An important simplification is that the contribution of the bound molecules can be handled by a free-energy preaveraged potential, which is simpler than the assumption of fixed relative orientation during binary collisions. Values for the initial pressure dependence of the viscosity of ammonia and stream are calculated starting from the angular-dependent Stockmayer potential and show satisfactory agreement with experiment.