A method of first principle calculation of alloy phase diagram was developed by the combination of first principle energy band calculation, cluster expansion method (CEM) and Monte Carlo (MC) simulation, where the effective multi-body potential energy for the flip test in MC simulation was obtained by the decomposition of the total energy by CEM. This method was applied to Cu-Au binary system. The calculated phase diagram agreed with that of CVM by introducing the dependence of the lattice constant on the concentration of the whole system. Furthermore an attempt of introducing the effect of local lattice relaxation was performed by the consideration of the local concentration. The order-disorder transition temperature became closer to the experimental value by adjustment of the local lattice constant depending on the concentration in the local region consisted of up to the second nearest neighbors of the atom tested for flipping.

A method for calculating the boundaries that describe solid-solid phase transformations in the phase diagrams of alloys is described. The method is first-principles in the sense that the only input is the atomic numbers of the constituents. It proceeds from the observation that the crux of the Monte-Carlo method for obtaining the equilibrium distribution of atoms in an alloy is a calculation of the energy required to replace an A atom on site i with a B atom when the configuration of the atoms on the neighboring sites, κ is specified, δHκ(A→B) = EB(κ)−EA(κ).Normally, this energy difference is obtained by introducing interatomic potentials, vij ,into an Ising Hamiltonian, but we calculate it using the embedded cluster method (ECM). In the ECM an A or B atom is placed at the center of a cluster of atoms with the specified configuration κ ,and the atoms on all the other sites in the alloy are simulated by the effective scattering matrix obtained from the coherent potential approximation. The interchange energy is calculated directly from the electronic structure of the cluster. The table of δHκ(A→B)'s for all configurations κ and several alloy concentrations is used in a Monte Carlo calculation that predicts the phase of the alloy at any temperature and concentration. The detailed shape of the miscibility gaps in the palladium-rhodium and copper-nickel alloy systems are shown.

A parameter-free approach to phase stability in substitutional alloys is applied to the influence of pressure on order-disorder phenomena in Al-Ge. The methodology is based upon an application of the Generalized Perturbation Method to the Korringa-Kohn-Rostoker scattering formulation of the Coherent Potential Approximation. For fcc-based Al-Ge alloys, it is shown that the tendency towards phase separation at normal pressure originates from a structural difference between the pure species. By applying pressure, the structural energy difference is reduced, and a significant increase in the tendency towards order, especially for Al-rich alloys, is theoretically observed, leading, as an example, to the possible observation of a DO22 ordered state around 25 at. pct. Ge. The electronic origin of the ordering tendencies induced by pressure is discussed and the theoretical predictions are related to experimental facts.

A tight-binding-bond approach to interatomic forces in disordered transition metal-Aluminium alloys is presented. The bond-order is calculated on a Bethe lattice reference system, well adapted to topologically disordered alloy. It is shown that the bond-order depends strongly on the strength of the pd hybridization in the AB alloy, leading to non additive potentials with a strong preference for the formation of pair of unlike atoms and short bond-distances in the A-B pairs. This is illustrated by studying the structural properties of liquid Al80Ni20 and Al80Mn20 alloys using molecular dynamics simulations and by comparing our results with the available experimental ones.

We study the kinetics of ordering in Cu3Au using a model Hamiltonian derived from the effective medium theory of chemical bonding. Monte Carlo simulations are used to investigate universal and non-universal features of the growth kinetics. Anisotropic scaling of the structure factor is observed in late-stage growth of ordered domains. The anisotropy is a non-universal feature determined by the details of the microscopic model, and we find that the anisotropy observed in the simulations is in excellent agreement with experiments on Cu3Au. The simulations are discussed in the context of theories of unstable growth. To our knowledge, this is the first study of kinetics in a realistic model Hamiltonian describing the material-specific properties of Cu3Au.

Although it is often convenient to perform computer simulations with small systems, in liquids the structure obtained using a small system may not be the same that would be obtained in the thermodynamic limit. In this paper we address the question specifically for the case of Stillinger-Weber silicon, identifying the region of density and temperature where small systems give different results from larger systems.

Tight-binding molecular-dynamics simulations are performed to study the structure of liquid and amorphous carbon. Comparisons of our results with ab initiomolecular dynamics (Car-Parrinello) results and experimental data show that the scheme has sufficient accuracy and efficiency for realistic simulation study of the structural properties of complex carbon systems.

Two real-space methods for treating structural energetics of transition metals and compounds are described. The first uses a local description of the electronic density of states (DOS) in a tight-binding model to obtain an angular-force method containing up to four-body interaction terms. It is shown that this method yields bond-strengthening effects at surfaces which exceed those obtained by previous many-body potentials. Structural-energy calculations show that for W, several Frank-Kasper phases are only slightly higher in energy than the ground-state bcc structure; this near-degeneracy is driven by the energetic favorability of icosahedral sites. The second method uses a free-electron type approach to generate pair potentials for transition-metal solutes in Al. The calculated potentials have an oscillating form, with a much larger magnitude than those in Al. The potential is applied to complex Al-Mn phases, including the icosahedral quasicrystal. The results indicate that the oscillating pair potentials make a major contribution to stabilizing the complex phases.

A scheme for construction of the pair potential from non-empirical calculations of electronic structure of solids is suggested. As an example, parameters of Lennard-Jones potential are obtained for fccCs, based on LMTO calculations of energy parameters. Vacancy formation and migration energies for fccCs are calculated from this first-principles pair potential. In addition, the frequency of vibration and the jump probability of an atom are calculated and it is shown that they are direction dependent.

In our first-principles-based molecular dynamics simulations for Si(100), we have found that the total energy U(θ) is rather flat with respect to the bond-rotation angle θ for |θ| <10°, with shallow minima at the positions θ = ± θ0 corresponding to a tilted-dimer geometry. Here we calculate the Fourier transform of the atomic displacements, u(ω), and find a characteristic bond-rotation mode with v − 5 THz. This mode is primarily associated with the upper atom in a dimer, and appears to be a signature of the tilted-dimer reconstruction. At low temperature the vibrations are nearly harmonic, but at high temperature there is strong anharmonicity, and θ appears to exhibit chaotic behavior.

We have explored the structure and the dynamics of a bimetallic system consisting of a hexagonal (almost) overlayer of Agon a square lattice (Ni(100) and Cu(100)), as a function of the surface temperature. In each case the structure is "nearly" incommensurate giving rise to a low frequency Goldstone mode. Also, the overlayer atoms slosh back and forth over the substrate in a corrugated fashion. The calculated dispersion of the Ag/metal vertical mode, at room temperature, is in excellent agreement with experimental data. At higher temperatures floater atoms appear on top of the overlayer displaying a variety of cluster formations and also exchanges with the substrate atoms leading to surface disordering, interdiffusion and melting.

We study ordering within closely packed monolayers of thiol derivatives self-assembled on gold substrates by chemisorption from solution. We discuss constant stress molecular mechanics and dynamics simulations of alkanethiolate monolayers on Au(111) and Au(l00) surfaces. In the first case, we use the MM2 force field augmented with classical chemisorption parameters obtained from fitting to results of ab initioquantum mechanical calculations. In the second, the chemisorption to the Au(l00) surface was accomplished by using harmonic constraints to restrict the thiolate groups to the square lattice. It is shown that two chemisorption modes of alkanethiolate on Au(l11) exist, and that both give closely packed ordered monolayers. In the system on Au(100), rearrangement towards close-packing is achieved at the expense of distortions that increase the intrachain elastic energy.

A new simulation method which combines the merits of Monte Carlo simulation of a lattice gas model and the continuum elasticity theory is described. This method treats the elastic strain energy due to concentration fluctuation of interstitial as a perturbation of a lattice gas model Hamiltonian. We illustrate this method by calculating the diffuse scattering intensity of YBa2Cu3O7 systems doped with trivalent impurity atoms M such as Fe or Al. The oxygen concentration wave amplitudes cq were obtained from Monte Carlo simulations of an anisotropic lattice gas model which represents well the interaction between oxygen atoms in this system. These results are in turn used to calculate the diffuse X-ray scattering intensity caused by the displacement field using concentration wave/displacement wave approach. The results suggest that the small orthorhombic domains associated with the oxygen “cross-link” around impurity atoms M causes the diffuse scattering intensity to fall off with oxygen concentration wave vector q as 1/q2for small qand as 1/q4 for larger q.We also show that the size of such domain can be obtained from diffuse scattering data.

A microscopic mechanism for the structural phase transition from the orthorhombic mediate temperature (OMT) phase into the low temperature tetragonal (LTT) one is suggested on the basis of the cooperative pseudo Jahn-Teller effect. The local distortions mixing the ground and the first excited electronic states are ordered antiferrodistortively and are connected in part, with the oxygen octahedra rotations around the [100] axis. The results are in agreement, with the neutron scattering experiments data.

Alumina is a common contaminate of titania, which is found in paints and food additives. Recently, thin films of rutile TiO2 grown on sapphire (α-Alumina) have received attention for possible use in electronic and optical devices.1−3 However, the alumina-titania interface has not been well characterized. In this work we report an extensive analysis of the alumina-titania interfacial relationship. Using existing structural data4, models of common alumina and titania surfaces have been constructed. The model used maximizes the commensurability between the two structures. Semi-empirical/tight binding calculations have been employed to examine the monolayer as well as the interfacial electronic properties in terms of chemical interaction potential and stability.

Structural and dynamic properties of a large class of intercalation compounds can be modelled by a collection of ions interacting via a screened Coulomb potential and subjected to the corrugation potential produced by the host atoms. In this paper we discuss the results of Molecular Dynamics simulation studies of the ground state structure of RbC24 and related systems focussing primarily on the micro-structure i.e.on the nature of the domains and domain walls.

Mordenite zeolites are well known for their acid cracking abilities, but are often hindered by coke formation which limits their life. The dealumination and generation of Bronsted acid sites in the MOR structure will have significant implications as to their catalytic activity. Density functional theory (DFT) has been applied to determine preferred aluminum siting in the Na-mordenite zeolite structure. DFT correctly reproduces the 6-31G* Hartree-Fock predictions for the relative stabilities of the four T sites of mordenite in a T(OH)4 cluster. Larger cluster sizes influence the preferred Al siting with DFT predicting T2 as the preferred tetrahedral site. Since DFT computational requirements are considerably smaller than Hartree-Fock theory, it is an attractive alternative for studying zeolites. The calculations performed were based on fixed geometries obtained from crystallographic data. Further work employing local geometry optimization in the vicinity of the substituted Al is underway.

The electronegativity equalization method is used to calculate the atomic charges, framework electronegativity and framework energy of the α–cristobalite bulk and the relaxed (011) surface. The effect of surface hydration and fluoridation on the framework properties are then calculated and related to surface charge. The segregation of germanium, aluminum and phosphorus in the α-cristobalite structure are modelled using the same method. The framework electronegativity difference between surface and bulk is observed to be a driving force for reactions and segregation.

We report some recent developments in theoretical modeling of real 3d magnetic materials, particularly interface and ultrathin-film structures. We focus on many-body correlation effects and describe the method of modeling realistic strongly correlated electron systems using and going beyond die local-density-approximation single-particle electronic structures. Group-theoretical techniques and high-performance computing facilities are systematically used in this work. Fundamental spectroscopic properties of several prototypical 3d transition-metal systems have been obtained and new features induced by many-body correlation have been discovered. Results will be presented and discussed.

We have partially solved the ground state problem of binary alloys on the fcc lattice with pair interactions up to the fourth nearest neighbor distance. Our results extend the study presented by Kanamori and Kakehashi [1], releasing the constraint they imposed on the nearest neighbor correlation. The solution we present increases the number of possible ground state structures by an order of magnitude with respect to previous results. We have applied both the polyhedron and the enumeration method. The latter proved more powerful when including interactions beyond the second nearest neighbor distance.