Showing papers on "Interatomic potential published in 1983"

The crystal structures of the elements: pseudopotential theory revisited

Abstract: The authors reconsider the trends among the crystal structures of the elements in terms of the real-space pair interaction. Experience with the on-Fermi-sphere approximation suggests that a simple empty-core pseudopotential is sufficient to explain the trends and indeed they find it so. Calculations of the interatomic potentials have been made for different core radius Rc, atomic radius Ta and valence Z. These are presented in a form to demonstrate the trends within groups I to V and within periods of the periodic table. They show that the structural trends arise from a characteristic variation of the real-space interatomic potential with electron density and pseudopotential. This more critical and detailed discussion now justifies and supplements the qualitative ideas advanced in the 1960s. An important effect not previously discussed stems from the disappearance of the Friedel oscillations as 2kFRc approaches 1/2 pi .

Computer simulation of the self‐sputtering of uranium

Abstract: The sputtering of polycrystalline α‐uranium by uranium ions of energies below 10 keV has been studied in the binary collision approximation using the computer simulation program marlowe. Satisfactory agreement of the computed sputtering yields with the small amount of available experimental data was achieved using the Moliere interatomic potential, a semilocal inelastic loss function, and a planar surface binding barrier, all with conventional parameters. The model is used to discuss low energy sputtering processes and the energy and angular distributions of the reflected primaries and the sputtered target particles.

Calculations of the properties of self-interstitials and vacancies in the face-centred cubic metals Cu, Ag and Au

Abstract: The properties of self-interstitials and vacancies in the noble metals Cu, Ag and Au were calculated using interatomic potentials derived entirely from first principles. Self-interstitials were found to be stable in the (100) dumbbell (or split) configuration. Their formation and migration energies were 2.61, 2.20, 3.81 and 0.11, 0.07 and 0.11 eV for Cu, Ag and Au, respectively. On the other hand, the respective energies of vacancy formation and migration were calculated to be 1.42, 1.37, 2.75, and 0.82, 0.54 and 0.71 eV. Good agreement between the present theoretical predictions and experimental measurements was obtained for Cu and Ag. In the case of Au, only the calculated vacancy migration energy was in good accord with experiment; the other quantities showed various inconsistencies with regard to experimental observations. For example, the results for self-interstitials could not explain the abnormality of the post-irradiation recovery stage I and interstitial clustering at about 5K observed experimentally in Au. Furthermore, the energy of vacancy formation was three times larger than the measured value. These discrepancies are ascribed mainly to the less reliable nature of the Au potential, relative to those for Cu and Ag, which results from the decrease in the justification for the applicability of perturbation theory. Calculations of the defect properties in Al, another face-centred cubic metal, for which the first-principles interatomic potential has been available, are also included for the purpose of comparison.

Point-defect calculations for tungsten

Abstract: Computer-model calculations have been carried out for vacancies, divacancies, and interstitials in tungsten using an interatomic potential developed by Johnson and White. Full relaxation of the 531 atoms closest to the defect is carried out by the model, and surrounding atoms are constrained to displace elastically. With an input into the model of a vacancy formation energy of 3.60 eV, the vacancy migration energy, formation volume, and migration volume were 2.00 eV, $0.79\ensuremath{\Omega}$, and $\ensuremath{-}0.08\ensuremath{\Omega}$, respectively ($\ensuremath{\Omega}$ is the atomic volume). The second-neighbor divacancy was most stable and migrated through partial separation to the fourth neighbor divacancy. The binding energy and volume were 0.78 eV and $0.08\ensuremath{\Omega}$, respectively, and migration was similar to that for single vacancies. The potential had to be modified for interstitial calculations to provide repulsion at near separations. For a hard repulsion, the formation energy was large (13.04 eV), the migration energy large (1.05 eV), and the formation volume small ($0.13\ensuremath{\Omega}$), while for a soft repulsion, the formation and migration energies were smaller (9.30 and 0.20 eV, respectively) and the formation volume was negative ($\ensuremath{-}0.65\ensuremath{\Omega}$). Although these calculations do not yield unambiguous results, they suggest that high-temperature curvature in self-diffusion data for tungsten is not caused by divacancies or by single interstitials.

Accurate Ne-heavier rare gas interatomic potentials

Abstract: Accurate interatomic potential curves for Ne-heavier rare gas systems are obtained by a multiproperty analysis. The curves are given via a parametric function which consists of a modified Dunham expansion connected at long range with the van der Waals expansion. The experimental properties considered in the analysis are the differential scattering cross sections at two different collision energies, the integral cross sections in the glory energy range and the second virial coefficients. The transport properties are considered indirectly by using the potential energy values recently obtained by inversion of the transport coefficients.

Computer Modeling of Cracks

Abstract: The theoretical techniques used in modeling cracks in crystalline lattices are reviewed. It is shown that there is generally a trade-off between sample size and realistic interatomic potentials. Infinite discrete one and two-dimensional lattices can be handled by the methods of lattice statics, but only with simple unrealistic potentials. In the hybrid lattice statics models a very small crystalline region, where the calculations are done with realistic potentials, is imbedded in an infinite elastic continuum. In this approach the boundary matching between the two regions is the difficulty. At the other end of the scale, molecular dynamic techniques can be used on an unconstrained system of a “large” number of atoms interacting with a reasonably realistic interatomic potential (this is the only way dynamic simulations have been done so far). Here, of course, the question is how “large” is large enough to simulate the behavior of the corresponding infinite system.

Rotational Rainbows in Atom-Diatom Scattering

Abstract: Systematic structures in cross sections are always very appealing for the scientist working in the field of molecular collisions. We mean those structures which vary in a predictable way as the parameters determining the collision conditions (i.e., energy, masses, scattering angle, coupling strength,...) are changed. The rainbow structures in the differential cross section and the glory oscillations in the integral cross section are important examples for isotropic potential scattering. Usually a deep physical understanding of the collision dynamics is gained from the detailed analysis of such structures which, in turn, makes the determination of the interatomic potential from measured quantities, in principle, feasible. This has been amply demonstrated for atom-atom scattering.

Many-body interactions in rare-gas solid mixtures

Abstract: The authors have proposed an interatomic potential model which takes account of many-body interactions arising from the variable induced dipoles, van der Waals attraction, overlap repulsion and zero-point energy in rare-gas solid mixtures. This potential has been employed to investigate their cohesive, harmonic and anharmonic elastic, thermodynamic and thermal properties. The theoretical results show rather good agreement with measured data on the host crystals and follow a systematic trend in the cases of mixed rare-gas solids.

Interatomic potential and phonon frequencies in α and β phases of lead fluoride

Abstract: Reports an interatomic potential for lead fluoride which is based on the shell model of lattice dynamics. The potential provides a good description of the experimentally determined dispersion curves in the beta (cubic) phase, the measured Raman frequencies in the alpha -phase and the formation energy of anion Frenkel defects in this solid. The authors suggest the use of this potential for studying other anion transport properties in PbF2.

Ab Initio Interatomic Potential Curves for NaNO2 and the Simulation of the Molten Salt

Abstract: Ab initio potential energy curves are presented for (Na + )(NO - 2 ) as a function of the Na–N separation. The minimum energy configuration , is similar to that observed in the room temperature ferroelectric solid. Empirical atom-atom potential energy functions based in part on the ab initio calculations, are used in a molecular dynamics computer simulation of the molten salt.

Interatomic Potential Structures in Highly Ionized Scattering Systems

Abstract: The interatomic potential of the ion-atom scattering system IN+-I at small intermediate internuclear distances is calculated for different charge states N from atomic Dirac-Focker-Slater (DFS) electron densities within a statistical model. The behaviour of the potential structures, due to ionized electronic shells, is studied by calculations of classical elastic differential scattering cross-sections.

Rigid muffin-tin model for interatomic forces in metals

Abstract: A rigid muffin-tin potential analysis is proposed for evaluating interatomic forces in metals. Using this theory, interatomic potential energy curves were calculated for Li, K and Rb. Despite the simple set of approximations used, these results are remarkably similar to the corresponding results obtained from pseudopotential theory. The rigid muffin-tin method is clearly useful for calculating interatomic forces in simple metals and should have a much wider application to all metallic systems.

Langevin equation — application to liquid state dynamics

Abstract: In these lectures I have introduced various relevant correlation functions which are essential in the study of atomic motion of liquids. The generalized Langevin equation has been discussed and procedure to make calculations from it is outlined. A mode coupling theory for the VAF has been discussed which quite successfully explains the experimental data, as well as known long time tail. Collective excitations are discussed within the framework of phenomenological theory. It is emphasized that the liquid Argon type system can be understood in terms of essentially one relaxation time, whereas for liquid metal type systems, two distinct relaxation times are essential. Recent microscopic calculations are also pointed out. In our opinion it would be of interest to relate the two relaxation times to microscopic processes and interatomic potential. In conclusion it can be fair to say that microscopic dynamics of liquid state is still an open problem.

Influence of Hydrogen on the Direction of Crack Propagation in Iron Using Computer Simulation

Abstract: The influence of hydrogen on the fracture behavior of α-iron was studied using computer simulation. The results indicate that the relative probability of (101) plane cleavage is enhanced compared with that for the (010) plane, although the lowest load for fracture is not decreased. Some limitations of the interatomic potentials used are discussed.