Showing papers on "Interatomic potential published in 1987"
TL;DR: In this paper, a simple accurate potential of the HFD-B form, which appears to be the best characterization of the He-He interaction constructed to date, is presented, and fitted to low temperature second virial coefficient data and recent accurate room temperature viscosity data, while at the same time pinning the repulsive wall to the value calculated by Ceperley and Partridge at 1 Bohr.
Abstract: A simple accurate potential of the HFD-B form, which appears to be the best characterization of the He-He interaction constructed to date, is presented. It has been fitted to low temperature second virial coefficient data and recent accurate room temperature viscosity data, while at the same time pinning the repulsive wall to the value calculated by Ceperley and Partridge at 1 Bohr. It possesses a well depth of 10·948 K, considerably deeper than many of the recent empirical or ab initio potentials. It reproduces, within experimental error, such dilute gas properties as second virial coefficients, viscosities and thermal conductivities over a wide temperature range. It also predicts, within experimental error, such microscopic properties as differential cross sections, high energy integral cross sections and backward glory oscillations in the integral cross sections. Finally, it accounts for nuclear magnetic relaxation in 3He and supports a weakly bound state in the 4He interaction.
405 citations
TL;DR: It is shown that the dynamic modelling technique that is used to obtain a structure in a polar solution from NMR distance information works well and yields an adequate representation of crystalline cyclosporin A.
Abstract: The conformation of the immunosuppressive drug cyclosporin A (CPA), both in apolar solution and in crystalline state, has been studied by computer simulation techniques. Three molecular dynamics (MD) simulations have been performed: one modelling the crystal structure and two modelling the structure in apolar solution, using a restrained MD approach in which data from nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy are taken into account. The simulation of the crystalline state (MDC) concerns a system of 4 unit cells containing 16 cyclosporin A molecules and 22 water molecules, which is simulated using crystalline periodic boundary conditions. The simulations modelling the apolar solvent conformation (MDS) concern one isolated cyclosporin A molecule. In these simulations an extra term in the interatomic potential function is used, which forces the molecule to satisfy a set of 57 atom-atom distance constraints originating from nuclear Overhauser effects (NOEs) obtained from NMR spectroscopy and one distance constraint deduced from IR spectroscopy. From a comparison of the results of the crystal simulation to those of the X-ray experiment in terms of structure, atomic fluctuations, hydrogen bond pattern, etc., it is concluded that the force field that is used yields an adequate representation of crystalline cyclosporin A. Secondly, it is shown that the dynamic modelling technique that is used to obtain a structure in a polar solution from NMR distance information works well. Starting from initial conformations which have a root mean square difference of 0.14 nm both distance restrained MD simulations converge to the same final solution structure. A comparison of the crystal structure of cyclosporin A and the one in apolar solution shows that there are significant differences. The overall difference in atomic positions is 0.09 nm for the C alpha atoms and 0.17 nm for all atoms. In apolar solution, the molecule is slightly more bent and the side chains of 1 MeBmt and 10 MeLeu adopt a different conformation.
67 citations
TL;DR: In this article, the authors describe recent work on the modeling of SiO2 and silicates using empirically derived effective potentials, and discuss briefly questions of methodology and review the different types of potential models that have been investigated.
Abstract: We describe recent work on the modelling of SiO2 and silicates using empirically derived effective potentials. We discuss briefly questions of methodology and review the different types of potential models that have been investigated. The greatest success appears to be enjoyed by shell model potentials which incorporate a ‘bond-bending’ term. Applications to the modelling of structural properties of zeolites, and to sorption in these materials are described; and we conclude with a summary of recent work on the defect structures of Mg2SiO4.
61 citations
TL;DR: The density-density response function of the homogeneous electron gas is shown to contain a part decaying monotonically as 1/r/sup 6/ in real space; this has consequences in the construction of generalized functional theories of interatomic potentials.
Abstract: This Letter is concerned with effective interatomic potentials in metals specifically arising from electronic quantum fluctuations. The cohesive energy is shown to contain large terms arising from generalized dispersion forces. Further, the density-density response function of the homogeneous electron gas is shown to contain a part decaying monotonically as 1/${\mathrm{r}}^{6}$ in real space; this has consequences in the construction of generalized functional theories of interatomic potentials.
57 citations
TL;DR: In this paper, the authors present experimental studies of the structure and dynamics of molten alkali and alkaline earth halides, which may be considered as completely ionic and without molecular or complex ions.
Abstract: Publisher Summary The chapter presents experimental studies of the structure and dynamics of molten alkali and alkaline earth halides. There are many different types of liquids, ranging from thixotropic materials to condensed inert gases; the physical properties and complexity may vary enormously. This chapter presents discussion on a relatively simple class of liquids-molten salts. The chapter discusses a subset of the class, molten alkali and alkaline earth halides, which may be considered as completely ionic and without molecular or complex ions. Nonmolecular liquids may readily be classified in terms of the interatomic forces. There are van der Waals forces in the condensed inert gases and longrange Coulomb forces in liquid metals and molten salts. The Coulomb forces in metals are strongly screened by the conduction electrons. The chapter explains characterizing the structure of the short-range order and relating this to parameters in the interatomic potential, given that the bonding within the alkali and alkaline earth group is similar and may therefore be parametrized in the same manner. The chapter explains the process of atomic motions that differ from those in comparable crystals with long-range ordering, and in the relation of this dynamics to the structure of the melt. The understanding of the structure and dynamics of the liquids provided by neutron and light scattering and computer simulation is discussed in considerable detail.
46 citations
TL;DR: The molecular structures of disiloxane, silanol and water have been determined by a local density functional method employing Gaussian bases as discussed by the authors, and the results were used to support the Griggs-Blacic-Frank models of hydrolytic weakening in quartz and to justify the use of an interatomic potential used in a previous investigation.
Abstract: The molecular structures of disiloxane, silanol and water have been determined by a local density functional method employing Gaussian bases. Bond lengths and angles are found to be within a few per cent of the observed values. A 4% expansion of the Si–O bonds in disiloxane is sufficient on energy grounds to allow hydrolysis to occur. These results are used to support the Griggs-Blacic-Frank models of hydrolytic weakening in quartz and to justify the use of an interatomic potential used in a previous investigation.
31 citations
TL;DR: In this article, the nuclear polarization is obtained by optical pumping at a wave-length λ = 1.08 μm with a LNA crystal laser, and the experimental results are in reasonable agreement with theoretical predictions, but the discrepancies are sufficiently clear to suggest that calculations should be slightly revised.
Abstract: Although it is a dilute nondegenerate system, a gas of 3He ↑ (spin-polarized 3He) can exhibit interesting quantum effects due to particle indistinguishability. One of these effects is a variation of the heat conduction coefficient as a function of the nuclear polarization. An experiment is described where this change has been observed and measured. The nuclear polarization is obtained by optical pumping at a wave-length λ = 1.08 μm with a LNA crystal laser. The experimental results are in reasonable agreement with theoretical predictions, but the discrepancies are sufficiently clear to suggest that calculations should be slightly revised (including, possibly, a new fit of the He-He interatomic potential).
21 citations
TL;DR: In this paper, the authors apply empirical interatomic potentials (Finnis-Sinclair potentials) to the calculations of the cohesive energy of transition-metal clusters of different sizes, in several structures.
Abstract: The authors apply empirical interatomic potentials (Finnis-Sinclair potentials) to the calculations of the cohesive energy of transition-metal clusters of different sizes, in several structures. The evolution of the cohesive energy and the structural parameters as a function of the cluster size is analyzed at T = 0. The validity of the Finnis-Sinclair potential for the structural determination of bulk solids and surfaces is also examined.
20 citations
TL;DR: In this article, a simulation was made for a screw dislocation in argon and xenon model crystals with various sizes, and it was found that the dislocation split into two Shockley partials, and the configuration of the partials depended on the boundary condition of the crystal surface.
Abstract: A simulation has been made for a screw dislocation in argon and xenon model crystals with various sizes. The number of atom rows in the crystal was 360-1400, and the Lennard-Jones (12-6) interatomic potential truncated at the third neighbors was adopted. The ordinary relaxation method was used to obtain the stable configuration of atoms in the crystal. It was found that the dislocation split into two Shockley partials, and the configuration of the partials depended on the boundary condition of the crystal surface. A method of modified boundary condition was proposed to control the effect of the surface. The split dislocations were able to stably exist when their separation was in a definite range, which was well understood by considering the balance of three forces: the interaction force between partials, the force due to stacking fault, and the image force from the crystal surface. By taking the limiting value of the separation distance for the crystal of infinite size, the stacking fault energy was esti...
15 citations
TL;DR: In this article, the condensed phases of the stable isotopes of helium, 3 He, and 4 He continue to be of high interest to scientists in general and to neutron scatterers in particular.
Abstract: Publisher Summary This chapter discusses that the condensed phases of the stable isotopes of helium, 3 He, and 4 He continue to be of high interest to scientists in general and to neutron scatterers in particular. This interest stems from the fact that these substances form the prototype examples in the nature of quantum solids and liquids of interacting bosons and fermions. A very meaningful comparison of the results of first-principles theoretical calculations, with experiment, is in fact possible for these materials, because the interatomic potential is reasonably simple and well known. Different regions of this potential can readily be probed in the experiments by varying the pressure and temperature to vary the interatomic distance, an extremely important advantage. The chapter highlights some of the most important features of the structure and excitations of the condensed phases of 3 He and 4 He; concentrating on the results from neutron scattering measurements, and on those theoretical developments that have direct consequences for neutron studies.
14 citations
TL;DR: In this article, the forces of interatomic interactions calculated from atom-atom potentials as -d V ij /d r ij allow us to interpret angular deformations in the ring of sodium p -nitrobenzoate trihydrate.
TL;DR: In this article, the monoclinic form of polyethylene under pressure up to 1.8 GPa at 298 K was investigated by x-ray diffraction, and the thermal expansion from 100 K to 300 K at atmospheric pressure was also studied.
Abstract: Lattice constants of the monoclinic form of polyethylene under pressures up to 1.8 GPa at 298 K are investigated by x-ray diffraction. The thermal expansion from 100 K to 300 K at atmospheric pressure is also studied. The equation of state is calculated by Pastine's method, taking into account the zero-point vibration, and is compared with experimental results. The calculated results depend largely on the interatomic potential used and the limit of the interatomic distance for summation of the potentials. A change in the angle γ between the a and b axes results in an anisotropic change in the unit cell dimension with both pressure and temperature; the a and b axes change at nearly the same rate. The monoclinic form may have a higher density than the orthorhombic form at pressures above 1.0 GPa.
TL;DR: In this paper, an accurate interatomic potential was constructed by fitting a realistic function, constrained by theory, to a set of judiciously selected macroscopic and microscopic properties but not including thermal conductivity.
Abstract: An accurate interatomic potential was constructed by fitting a realistic function, constrained by theory, to a set of judiciously selected macroscopic and microscopic properties but not including thermal conductivity Using this potential, accurate values of thermal conductivity of argon are obtained for the temperature range 100 to 6600 K These are presented in tabular form as well as in terms of a correlation function
TL;DR: In this article, stick and ball models of As 2 S 3 were computer-relaxed in an interatomic potential consisting of bond bending, bond stretching, lone-pair and core repulsion terms.
Abstract: Stick and ball models of As 2 S 3 were computer-relaxed in an interatomic potential consisting of bond bending, bond stretching, lone-pair and core repulsion terms. The RDF and pair distribution functions calculated from the relaxed models show good agreement with experimental data. The relaxed models exhibit helical ordering without indications of planar structures.
TL;DR: In this paper, the sensitivity of the d-electronic structure of an amorphous system to various parameters of the structural model is investigated, including the dependence on the size of the model, the effect on the resulting DOS of the periodic boundary conditions, the dependence of DOS on the thermodynamic state of the system (temperature, density) and on the parameters of Morse interatomic potential used in the structural simulation.
Abstract: Computer modelling of the atomic structure of real amorphous systems meets the serious problem of our vague information about the effective interatomic interactions. The uncertainty about the atomic structure is projected into the results of the electronic structure calculations. Here the sensitivity of the d-electronic structure of an amorphous system to various parameters of the structural model is investigated. The dependence on the size of the structural model of the density of d-electronic states (DOS), the effect on the resulting DOS of the periodic boundary conditions, the dependence of DOS on the thermodynamic state of the system (temperature, density) and on the parameters of the Morse interatomic potential used in the structural simulation are examined. Particular attention is paid to the dependence on atomic configurations of the d-electronic structure. The structural models were generated by means of thermodynamic simulation methods. The electronic structure was calculated by the recursion method. A model of 256 atoms with a periodic boundary condition appears to be sufficiently large to provide the basic features of the d-electronic structure of an amorphous system. The shape of the DOS curve is most sensitive to those parameters of the structural model which affect the interatomic distances and the local arrangement of the atoms, and in particular to the repulsive part of the interatomic potential. The configuration fluctuations of the total-one-electron energy of the 256-atom model of an amorphous system at 300 K are less than 0.01%.
TL;DR: In this article, the total differential scattering cross section of Ar+-Ar at laboratory energies between 15 and 400 keV was measured using an ab initio relativistic molecular program which calculates the interatomic potential energy curve with high accuracy.
01 Nov 1987-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: The shadow cone radii of 1 to 3 keV H +, He +, Li +, Ne + and Na + are calculated by classical scattering theory using the universal potential of Biersack and Ziegler.
Abstract: The shadow cones of 1 to 3 keV H + , He + , Li + , Ne + and Na + scattered from a variety of target atoms ranging from C to Pb are calculated by classical scattering theory using the “universal potential” of Biersack and Ziegler. We compare these shadow cone radii with those calculated from using the Thomas-Fermi-Moliere interatomic potential and also with experimentally determined values.
TL;DR: A general method is described for the calculation of interatomic potentials in condensed-matter systems by use of a maximum-entropy Ansatz for the interatomic correlation functions.
Abstract: A general method is described for the calculation of interatomic potentials in condensed-matter systems by use of a maximum-entropy Ansatz for the interatomic correlation functions. The interatomic potentials are given explicitly in terms of statistical correlation functions involving the potential energy and the structure factor of a ``reference medium.'' Illustrations are given for Al-Cu alloys and a model transition metal.
TL;DR: In this article, it was shown that if the ratio of the numbers of atoms of two types is irrational, then the state of absolutely minimal energy is quasicrystalline.
Abstract: A one-dimensional chain of atoms of two types is investigated. It is proven exactly for the model of attracting hard spheres that if the ratio of the numbers of atoms of the two types is irrational, then the state of absolutely minimal energy is quasicrystalline. The quasicrystalline state is also investigated in the case of the Lennard-Jones interatomic potential. All the microscopic values (interatomic spacing, electronic density, etc.) are shown to be quasiperiodic functions varying on Cantor sets. Diffraction patterns, electronic properties, and vibrational spectra are also discussed.
01 Nov 1987-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this article, the effect and relative importance of the changes in the interatomic potential and electronic stopping power are calculated, and simple estimates of the effect on the mean range are given.
Abstract: The capabilities of various methods in calculating range distributions are discussed. The effect and relative importance of the changes in the interatomic potential and electronic stopping power are calculated, and simple estimates of the effect on the mean range are given. The influence of various crystalline structures on the distribution is calculated. Some other factors affecting range distributions are briefly mentioned.
01 Jan 1987
TL;DR: In this article, the ground-state energy of the system of electrons and ions as a function of the ionic coordinates was calculated starting from the full electron-ion Hamiltonian given by (1.1).
Abstract: Our first task will be the calculation of the ground-state energy of the system of electrons and ions as a function of the ionic coordinates {R l }, starting from the full electron-ion Hamiltonian given by (1.1). Furthermore, we shall seek an expansion of E({R l }) in terms of a volume energy and sums over pair, triplet and higher-order interactions.
01 Jan 1987
TL;DR: In this article, high-resolution crossed molecular beam measurements of diffraction oscillations for some atom-atom and atom-diatom systems in the thermal energy range are reported.
Abstract: High resolution crossed molecular beam measurements of diffraction oscillations for some atom-atom and atom-diatom systems in the thermal energy range are reported. From the observation of a well resolved diffraction structure in the Ne-Ne, Ar, Kr and Xe systems a refinement or assessment of the corresponding interatomic potentials to better than 1% has been obtained. From the simultaneous best-fit, within the infinite-order-sudden approximation, of the diffraction data, and of absolute total integral cross sections and second virial coefficients from literature, rather accurate potential energy surfaces for the He-N2, O2 and NO systems have been derived.
TL;DR: In this paper, the broadening and shift parameters for a number of Fei lines perturbed by atomic hydrogen are computed using the interatomic potential due to Hindmarsh et al. (1967, 1970).
Abstract: The broadening and shift parameters for a number of Fei lines perturbed by atomic hydrogen are computed using the interatomic potential due to Hindmarsh et al. (1967, 1970). It is also shown that the rms radius and the effective radius of the radiating atom, which determine the force constants in the interatomic potential, can be simply related each other, depending on the orbital quantum number of the atomic level.
TL;DR: In this paper, a mixed mechanical-computer model was used to obtain information about the medium range order in amorphous As2S3, and the model showed evidence of polymeric nonplanar structural ordering.
Abstract: A mixed mechanical-computer model was used to obtain information about the medium range order in amorphous As2S3. Beginning with a mechanical stick and ball model, the structure was computer relaxed with an interatomic potential. The relaxed model shows evidence of polymeric non-planar structural ordering.
TL;DR: In this paper, the volume dependence of the frequency is defined in terms of the Gruneisen parameter 7, which is a measure of the volume of the potential energy of the crystal lattice.
Abstract: The thermal expansion of a solid body arises because of the anharmonicity of the potential energy of a crystal and is directly related to the motion of the atoms in the interatomic potential of the crystal. Therefore, thermal expansion is considered to be a source of information on a measure of the anharmonicity of the interatomic potential and on the atomic vibrations in the crystal lattice. Thermal expansion is usually studied by means of a quasiharmonic approximation because this approximation neglects anharmonic terms and considers the ionic vibrational frequency to be volume dependent, thus accounting, at least in part, for the anharmonic effects. The volume dependence of the frequency is defined in terms of the Gruneisen parameter 7, which is a measure of the
TL;DR: In this paper, the relation between the spectral character of a completely off-resonant two-photon absorption and the non-impact collision dynamics of a three-level atomic system is studied.
Abstract: The relation between the spectral character of a completely off-resonant two-photon absorption and the non-impact collision dynamics of a three-level atomic system is studied. A collisionally induced resonance character in the spectral dependence of the absorption probability is shown theoretically. Measured results about the dependence of fluorescence intensity on the incident light frequencies are ingood agreement with the theory. The interatomic potential between the sodium 4D state and argon is also estimated from experimental results.
TL;DR: In this article, it was shown that there is a sizable attractive short-range component of the interatomic potential due to the s-d interaction which has the same functional form in real space as the Born-Mayer repulsion due to overlap of core electron wave functions centred on neighbouring ions.
Abstract: It is shown in the framework of the pseudopotential approach that there is a sizable attractive short-range component of the interatomic potential due to thes-d interaction which has the same functional form in real space as the Born-Mayer repulsion due to the overlap of core electron wave functions centred on neighbouring ions. The magnitude of this attractive component is such as to completely cancel the conventional Born-Mayer repulsion, making the resultant short-range interatomic potential attractive rather than repulsive. Numerical calculations show that the attractive interatomic potential, which represents the local-field correction, leads to a better understanding of the occurrence of the soft modes in the phonon dispersion curves of niobium and tantalum.
TL;DR: Ashcroft empty core pseudopotentials and Ichimaru and Utsumi screening are used to calculate interatomic pair potentials which reproduce the measured entropies of some liquid and hot solid metals.
Abstract: Ashcroft empty core pseudopotentials and Ichimaru and Utsumi screening are used to calculate interatomic pair potentials which reproduce the measured entropies of some liquid and hot solid metals. It is found that the empty core radii vary rather continuously with volume from phase to phase. This variation can, however, be rather strong in some cases (e.g. Na, Mg) if not in others (e.g. Al). Such a possibility should be considered, therefore, whenever volume changes are significant and, in particular, across solid-liquid phase boundaries. In Na and Mg, the interatomic force appears to be at least semi-quantitatively given over the small region between the effective collision diameter in the liquid and the nearest neighbour distance in the solid; elsewhere, it is scarcely tested by the fitting exercise. In Al and Ge, the analysis is consistent with a loss (partial or total) of minimum of the pair potentials in the nearest neighbour regions. The importance of these regions for the solid state analy...
01 Jan 1987
TL;DR: In this article, anharmonic vibrations of atoms in a micro-cluster are theoretically studied by using the self-consistent phonon formalism and the free energy of a cluster is minimized by using a model reference system in which a central atom is vibrating with a frequency ω 0 and the surrounding atoms with ω 1.
Abstract: Anharmonic vibrations of atoms in a micro-cluster are theoretically studied by using the self-consistent phonon formalism. Atoms are assumed to interact on each other through a pairwise interatomic potential. The free energy of a cluster is minimized by using a model reference system in which a central atom is vibrating with a frequency ω0 and the surrounding atoms with ω1. We investigate the thermal softening of atomic vibrations, the thermal expansion and the vibrational instability for different micro-clusters. As for N=13 clusters, Cuboctahedron becomes instable at lower temperature than Icosahedron. The thermal stability of micro-clusters is discussed in terms of the instability temperature.