Showing papers on "Interatomic potential published in 1985"

Interatomic potentials for silicon structural energies.

Abstract: We develop two- and three-body classical interatomic potentials that model structural energies for silicon. These potentials provide a global fit to a data base of first-principles calculations of the energy for bulk and surface silicon structures which spans a wide range of atomic coordinations and bonding geometries. This is accomplished by use of a new "separable" form for the three-body potential that reduces the three-body energy to a product of two-body sums and leads to computations of the energy and atomic forces in ${n}^{2}$ steps as opposed to ${n}^{3}$ for a general three-body potential.

Vibrational spectroscopy in the mineral sciences

Abstract: The atomic vibrations of a molecular system are governed by the interatomic potential determined by the type and relative arrangement of its constituent atoms. At the same time, many of the bulk thermodynamic and material properties of the system are related directly or indirectly to its vibrational nature; hence a study of vibrational properties can form a true link between the microscopic and macroscopic characteristics of the material. The purpose of the present chapter is to introduce some of the basic concepts and terminology of vibrational theory and spectroscopy, describe certain aspects of experimental infrared and Raman spectroscopy, and discuss some of their applications to problems in geochemistry and mineralogy. Refs.

Calculation of elastic constants by the method of crystal static deformation

Abstract: The contribution of homogeneous lattice deformations (neglecting internal strains) to elastic properties of crystals with triclinic or higher symmetry is examined. The deformed lattice constants are expressed as functions of the components of the finite Lagrangian strain tensor, and their derivatives are calculated. Thus equations are obtained that relate the second-order elastic constants to first and second partial derivatives of the static crystal energy with respect to unit-cell parameters. With the assumption of a two-body Born-type interatomic potential, the energy derivatives were calculated analytically, and a rigid-body approximation was introduced to account for molecular groups in the crystal structure. Test computations of elastic constants were performed for MgF2 (rutile-type), benzene and naphthalene, using literature potential parameters optimized on structural data; results are discussed with respect to adequacy of the potentials and of the approximations of the model used.

First-principles interatomic potentials in transition metals

Abstract: The first ab initio theory of structure-independent interatomic potentials in $d$-electron transition metals has been developed from a multi-ion expansion of the total energy within the density-functional formalism. Explicit results for the volume term and two-ion and three-ion potentials entering this expansion have been obtained and successfully tested for $3d$-series metals.

Models of glass strength and relaxation phenomena suggested by molecular dynamic simulations

Abstract: A model is proposed to explain several properties of MD ensembles representing silica and silicate glasses. The model involves the breaking of bridging SiO bonds. The breaking of a bond is here defined as stretching the bond to an inflection point in the interatomic potential. This model is proposed to explain: (1) the theoretical or intrinsic strength of the glasses, (2) flow and stress and structural relaxation, (3) melting and devitrification, (4) why silica based compounds form thetrahedral network glasses easily whereas other analogous compounds, AlPO4 for example, does not and (5) the thermal expansion of silica. The model is not new and only suggested by the molecular dynamic calculations, that is, we have not been able to observe the mechanism of glass failure and flow in the simulations. More extensive analysis and simulations are needed in the future.

An effective approach for elastic scattering description in Monte Carlo simulation

Abstract: An effective approach of calculating scattering angle and time integral in Monte Carlo simulation is described for the Ziegler et al. interatomic potential. The scattering angle is determined using the “magic” formula by Lindhard et al. and that by Biersack et al. in the corresponding regions of p/a and ϵ variation. To calculate the time integral, a simple and rather precise approximation formula was obtained.

Effective interatomic pair potentials in liquid polyvalent metals from observed structure data

Abstract: A method which is based on the random phase approximation (RPA) and Monte Carlo (MC) computer simulation is applied to liquid Mg, Zn, Ca, Sr, Ba, Al, In and Pb to extract the effective pair potential nu eff(r) using the experimental structure factor a(k) near the melting point. For Zn, Al, In and Pb the nu eff(r) thus obtained have a markedly raised principal minimum, while for Mg, Ca, Sr and Ba they display a conventional attractive well at the first-neighbour distance. They present results, though not fully quantitative, give good support to the recent analysis by Jacucci and Taylor (1981) in which one expects the interatomic potential to be repulsive at the first-neighbour distance for simple metals with a high density of conduction electrons.

Total scattering cross sections and interatomic potentials for neutral hydrogen and helium on some noble gases

Abstract: Measurements of energy‐dependent scattering cross sections for 30 to 1800 eV D incident on He, Ne, Ar, and Kr, and for 40 to 850 eV He incident on He, Ar, and Kr are presented. They are determined by using the charge‐exchange efflux from the Princeton large torus tokamak as a source of D or He. These neutrals are passed through a gas‐filled scattering cell and detected by a time‐of‐flight spectrometer. The cross section for scattering greater than the effective angle of the apparatus (≈20 mrad) is found by measuring the energy‐dependent attenuation of D or He as a function of pressure in the scattering cell. The interatomic potential is extracted from the data.

Interpretation of rare-gas-induced broadening of Rydberg series lines

Abstract: Approximate expressions have been derived for the cross-sections of rare-gas-induced broadening of the Rydberg series lines on the basis of the interatomic potential which is expressed as a combination of the van der Waals potential, the Fermi potential and the polarization potential. Only the effect of elastic scattering has been taken into account. Multichannel (two-channel) quantum defect theory has been combined with the broadening theory to include the effect of the mutual interaction of the Rydberg series. The resulting cross-sections are in reasonable agreement with the experimental values obtained for Na by Kachru et al. and for Ca and Ba by Ueda et al.

Predicting transport properties without adjustable parameters: A test application of the Hulburt-Hirschfelder potential to argon

Abstract: Accurate estimates of the transport properties of gaseous systems under conditions where experimental transport data are sparse of unavailable are important in a number of applications. The Hulburt-Hirschfelder (HH) potential for monatomic gas interactions, which is determined entirely by spectroscopic constants of diatomic molecules, provides a basis for calculating transport properties without adjustable parameters. In this paper we report test calculations of the viscosity, thermal conductivity and self-diffusion coefficients for argon. Comparison with the comprehensive correlation of thermophysical properties for argon by Kestin and co-workers shows very reasonable agreement for the transport properties at moderate and high temperatures. Deviations at lower temperatures may be attributed to inaccuracies in the long-range part of the HH potential, whereas the core and well of the potential appear to be adequately represented. These results strongly support the use of the HH potential for estimating the transport properties of monatomic gases at high t

Theory of freezing in simple systems

Abstract: The transition parameters for the freezing of two one-component liquids into crystalline solids are evaluated by two theoretical approaches. The first system considered is liquid sodium which crystallizes into a body-centered-cubic (bcc) lattice; the second system is the freezing of adhesive hard spheres into a face-centered-cubic (fcc) lattice. Two related theoretical techniques are used in this evaluation: One is based upon a recently developed bifurcation analysis; the other is based upon the theory of freezing developed by Ramakrishnan and Yussouff. For liquid sodium, where experimental information is available, the predictions of the two theories agree well with experiment and each other. The adhesive-hard-sphere system, which displays a triple point and can be used to fit some liquids accurately, shows a temperature dependence of the freezing parameters which is similar to Lennard-Jones systems. At very low temperature, the fractional density change on freezing shows a dramatic increase as a function of temperature indicating the importance of all the contributions due to the triplet direction correlation function. Also, we consider the freezing of a one-component liquid into a simple-cubic (sc) lattice by bifurcation analysis and show that this transition is highly unfavorable, independent of interatomic potential choice. The bifurcation diagrams for the three lattices considered are compared and found to be strikingly different. Finally, a new stability analysis of the bifurcation diagrams is presented.

Computer simulation of grain-boundary properties in ionic crystals and metals

Abstract: In recent years a set of computer codes has been developed at Argonne which permits one to determine the energies, structures, and point-defect properties of interfaces in metals and ionic crystals. To demonstrate some of the capabilities of these codes, in this article the calculated energies and structures of high-angle 〈100〉 twist grain boundaries in fcc metals and in metal oxides with NaCl structure are compared. The role of impurities and Schottky defects on the stability of such boundaries in the metal oxides is discussed in some detail. Particular emphasis is placed on the effect of the interatomic potential used in our calculations on the predicted physical properties of these grain boundaries. We conclude with a summary of those properties which depend on the potential chosen (i.e. on the particular material considered) and those phenomena which are independent of the potential.

Dispersion of surface phonons in xenon overlayers physisorbed on the Ag(111) surface

Abstract: Computer simulation and lattice dynamics have been used to investigate phonons propagating in monolayers, bilayers, and trilayers of xenon physisorbed on a Ag(111) surface. Agreement with the experimentally determined dispersion of the surface phonons has been achieved by the use of a realistic interatomic potential for the Xe adatoms and a new surface-adatom potential.

The static approximation in the interpretation of collision broadening

Abstract: The validity of the static approximation for analyses of spectral line profiles is considered. The Fourier transform of the profile of an isolated line is compared for representative cases with the unified theory, and it is shown that the simple static approximation can be used effectively to interpret low spectral resolution experiments. This result applies to the entire profile in the limit of high density, but it also applies at low density if the instrumental line width is sufficiently large. Furthermore, in the absence of turning points in the interatomic potential, the unified and static theories produce almost identical line-wing profiles. Easily evaluated criteria are provided to guide application of the straightforward static theory to the interpretation of interatomic potentials.

Molecular dynamics studies of the coherency of monolayer films

Abstract: The equilibrium configurations of monolayer films on crystalline substrates have been investigated using molecular dynamics techniques. Misfit dislocation densities are found to differ from those predicted by elasticity theory, because of the more realistic interatomic potential employed in the simulations (Lennard-Jones), and the absence of constraints on atomic positions. Two distinct dislocation core structures are observed, with a discontinuous transition between the two as the external conditions are changed. Shockley partial dislocations are found to be stable for a large range of misfits in the case of FCC (001) substrates, and recently an experimental observation of these dislocations was reported. The stability of monolayer films relative to the formation of three-dimensional clusters is also considered.

Bound state properties of the Lennard-Jones 8-6 potential

Abstract: A numerical method is used to calculate the vibrational energy eigenvalues for the Lennard-Jones 8-6 potential. Values are given for the excimers NaAr, NaKr, NaXe, KNe, KAr, KKr, and KZe. Systematics of the eigenvalues are studied for three values of β, where β is a dimensionless parameter which is a measure of the strength of the interatomic potential. Birge-Sponer plots are made and it is shown that a plot of β 1 2 ΔG v+ 1 2 versus v′ β 1 2 , where v′ is the quantum number of the upper level, yields a “universal” curve which is almost independentt of β.

Refined potentials for rare gas atom adsorption on rare gas and alkali-halide surfaces

Abstract: The utilization of models of interatomic potential for physical interaction to estimate the long range attractive potential for rare gases and ions is discussed. The long range attractive force is calculated in terms of the atomic dispersion properties. A data base of atomic dispersion parameters for rare gas atoms, alkali ion, and halogen ions is applied to the study of the repulsive core; the procedure for evaluating the repulsive core of ion interactions is described. The interaction of rare gas atoms on ideal rare gas solid and alkali-halide surfaces is analyzed; zero coverage absorption potentials are derived.

Thermodynamic properties of anharmonic noble metals in terms of interatomic potentials from experimental phonon spectra

Abstract: A new model interatomic pair potential derived from the experimental phonon spectra is applied to the investigation of the thermodynamical properties of anharmonic noble metals.

Effects of the T-matrix center-of-mass approximation in the Brueckner–Sawada theory of liquid helium II

Abstract: As a model for liquid helium II, we study the Bose–Einstein gas with a two-body interaction potential of the form ~δ(r − a), where r is the interparticle separation. Excitation spectra for various ...

Pressure dependence of the compressibility of alkali halides

Abstract: The parameterC1=[∂(1/KT)/∂P]T, which describes the pressure variation of the compressibility, has been examined correlating the thermodynamical and interatomic potential approaches employing fewer approximations than has been usual heretofore. General expressions have been derived forC1 by including the thermal correction terms, which have generally been ignored in previous studies concerning thermal properties of ionic crystals. The parameterC1 has also been related to the Gruneisen parameter, γ, using a relation given earlier. The applicability of the derived equations is investigated and discussed for alkali halides employing few realistic potential forms. A good general accord is found with the available experimental data, which exhibits an essential improvement over other theoretical determinations.