In molecular-dynamics simulations for the long-range Coulomb interaction, a great deal of effort is devoted to reducing the computational complexity of the usual N2 operations in the direct calculation. For bulk systems, we have designed a parallel algorithm based on the domain-decomposition strategy for the Ewald summation. The performance of the algorithm is evaluated on the in-house iPSC/860 system. We find that this algorithm reduces the computational complexity to O(N). For a 64,000-particle plasma in three dimension, the execution time on an 8-node system is 27.4 sec per MD time step. The interprocessor communication is a small fraction of the total execution time. We find linear speedups and a parallel efficiency of 0.85. For comparison, parallel algorithms are also designed for the Fast Multipole Method (FMM) - a divide and conquer scheme in which the system is divided into cubic subdomains and interactions between distant charged regions are calculated with a truncated multipole expansion. The performance of the FMM on Touchstone Delta machine is discussed.

The role of statistical modelling in materials research is examined. It is shown that considerable benefits can be derived by following an appropriate full factorial design, in which a number of factors (process parameters) are varied simultaneously while the responses (materials properties) are monitored. It is also shown that by using a fractional factorial matrix, essentially the same conclusions can be drawn with a significantly smaller number of experimental trials. Examples drawn from recent materials research on glasses and ceramics are presented to demonstrate the effectiveness of this approach.

A new method to solve the extended Hubbard Hamiltonian for systems with few electrons is reported. This method is based on mapping the original many-body problem onto a tight-binding one in a higher dimensional space, which can be solved exactly. For one-and two-dimensional periodic lattices, the real-space pairing problem of two electrons with parallel and anti-parallel spins is analyzed by looking at the binding energy, the coherence length and the mobility of electron pairs. Likewise, some results of the three-body correlation are also reported.

Distortion and rotation of a molecular dynamics cell used in Parrinello-Rahman molecular dynamics are found to lead to slow convergence, or nonconvergence of fluctuations from thermodynamic averages. The variations are shown to be related to nonconservation of the total angular momentum and translational symmetry variance of the dynamics. A modified equation of motion is presented which eliminates these variations. It is shown that the ergodicity is achieved in the MD ensemble generated by the new equations of motion. However, the rate of convergence is strongly affected by the choice of the MD cell mass W. Simulation results show that not all values of Wcan be used to give a desired convergence of fluctuations from thermodynamic averages in finite simulations. The fastest convergence is achieved by using the optimal cell mass.

We have calculated the binding energies of several two-electron pseudoions using the Diffusion Quantum Monte Carlo method. The comparison between our results and the experiment suggests that HSC pseudopotentials are portable from the original single particle theory (DFT-LDA) to the “exact” many body one. Moreover we are able to evaluate the degree of portability of each effective field.

Using molecular dynamics simulation we have investigated the melting properties of a 2-dimensional system of ions interacting through a screened Coulomb potential in the presence of a substrate. We focus on the role of the substrate potential in the melting transition. The physical system we study is the stage-2 graphite intercalation compound RbC24 and a related system RbC24,57 whose intercalant density differs from the former by a small amount. Our results reveal the nature of melting processes characteristic of periodic domain wall solids.

Using molecular dynamics simulation, we have studied the dynamics in the fluid phase of ionic overlayers on corrugated substrates. This is a good model to probe the dynamics of intercalants inside the galleries of intercalated systems. We have calculated the diffusion constant from the incoherent dynamic structure factor, de Genne oscillations in the q-dependence of the Rayleigh peak width and dispersion of Brillouin peaks have been obtained from the coherent dynamic structure factor. These results are compared with neutron scattering measurements in stage-2 RbC24 and earlier simulation results.

The GaN(OOl) surface is polar with either N- or Ga-termination. We predict that the Ga-terminated surface does not dimerize, but instead the surface Ga atoms relax into the vacuum by about ≈0.38 Å. The N-terminated surface is predicted to form a c(2×4) structure with N2 dimers in rows of length two. This is to be contrasted with the (2×1) Si(001) surface, on which the dimer rows are infinitely long, and with the (2×4) GaAs(OO1) surface, on which the rows are three dimers long.

The ultrathin (GaAs)m (AlAs)n superlattices ( SL ) grown along [001] are formally considered as a result, of ordering-type phase transition from an initial hypothetical mixed crystal (Gax Al1-x)As with x = m/(m+n) . From this point of view changes of elastic modulus tensor (EMT) induced by ordering can he calculated nearly in a spirit of phenomenological Landau theory of second-order phase transitions. It .is shown that a change of EMT component. S66 is completely determined by symmetry properties of two translational (Dzyaloshinskji) invariants of m+n power .in the expansion of SL thermodynamic potential in a power series .in order parameter components. The changes of the other EMT components depend on the second power of order parameter .The expressions are found out to connect. EMT of a definite ordered phase (SL - family with m+n = 3) with that, of the initial one.

Light weight, high strength fibers and films produced from stiff chain polymers are good candidates for use as structural materials. Over the last decade, considerable success has been achieved in synthesizing high strength fibers and films. Due to their thermal and oxidative stability aromatic heterocyclic stiff chain polymers such as ABPBO, ABPBT, and ABPBI are especially good candidates. We first describe the finite theory of elasticity as applied in atomistic modelling and simulations of anisotropic solids and then use this description to investigate the mechanical response of these crystalline polymers as a function of applied hydrostatic pressure and uniaxial tension and compression along the chain direction in molecular mechanics simulations. In addition to these finite stress-strain experiments, I will also present the results of the first elastic stiffness matrix calculations performed on these high performance polymers.

An electrostatic induction model based on atom-centered partial charges is presented. In this representation, the local electric field is evaluated at each bond center. The bonds are treated as entities polarizable only in a direction parallel to the bond itself by means of a scalar bond polarization parameter. The resulting induced charge separation contributes to the local electric field experienced by neighboring bonds, resulting in a final set of charges which are calculated self-consistently. The model provides a reasonable approximation to dipole moment vectors and polarizabilities of small molecule fluoroalkanes. The results are discussed in comparison with other induction models. This model fits well into the framework of established atomistic potential energy calculations using an atom-centered partial charge representation.

The electronic structure ofLaBO3 perovskites and La2NiO4 perovskite- like materials (where B = Mn,Fe,Co,Ni,Cu) have been studied inside a tight binding scheme using the extended Hückel Method. An analysis of the metal- oxygen bond is presented as well as the electronic structure in order to establish a correlation between electronic properties and catalytic activity (oxidation) of these compounds.

We use the continued fraction formalism (CFF) to obtain the zz- component of the dynamical spin pair correlations of a transverse Ising model (TIM) in 2D at T = ∞ for the first time. Our calculations suggest that the correlation function decays algebraically at long times as t−α, 1.5 < α < 2.2, which is slower than gaussian decay found for the corresponding dynamical correlation in 1D. The calculations suggest that the corresponding dynamical correlation in 3 D is also likely to exhibit a power law decay. Our results compare favorably with the neutron scattering data on LiTbF4, for T > Tc, an induced moment ferromagnet.

We study the atomic vibrational dynamics in silica glass (a-SiO2) using molecular-dynamics (MD) simulations and classical lattice dynamics method. The SiO2 glasses were generated by molecular-dynamics and steepest-descent quench (SDQ) using an effective interatomic potential consisting of two-body and three-body interactions. The frequency and eigenvectors of vibrational normal modes are obtained by diagonalization of the dynamical matrix. The partial and total vibrational density of states (DOS), bond-projected DOS, participation ratio (PR), and neutron-weighted dynamic structure factor are calculated. The results are compared with inelastic neutron scattering experiments on SiO2 glass.

The electronic structure of random clusters has been used in the literature as representative of the electronic structure of random solids. In this work a calculation of the local density of states (LDOS) and charge density contours for clusters of the type XSi20H28 with X an Si atom, a vacancy or 4 hydrogen atoms, has been carried out. The method used was a pseudopotential SCF Hartree-Fock and the HONDO program. It is found that the generation of a vacancy in the center of the cluster (removal of the central Si atom), introduces p-like states in the energy gap of the LDOS for the region near the center of the cluster. The saturation of the dangling bonds of the vacancy with 4 hydrogen atoms removes the states within the gap. These results are also borne out by the charge density contours, thereby reinforcing the importance of amorphous cluster calculations in the understanding of the electronic structure of amorphous solids.

We report our results of molecular structure calculations for B and A1 impurities. From our unoptimized ab initio calculations, we found the molecular electronic wave function to be unstable for both impurities. This instability was removed through a geometry optimization process. Local densities of states (LDOS) were computed for the optimized geometries. They show a rise of a peak at the tail of the valence LDOS; this feature is due to p orbitals of B and Al. The contribution is slightly higher for B than for A1 impurities. Charge contour plots are presented.

Computer simulations were performed to investigate the migration energy of vacancies in A1 in the presence of grain boundary, dislocation and various imposed forces (such as stress gradients and electro-migration). The energy values were significantly smaller when the vacancy moved into the grain boundary or a dislocation core than in opposite direction. This makes these defects sinks for vacancies from which they are unlikely to migrate out. The major time required for the movement of vacancies is thus limited to within a single grain. Calculations of migration time from the center of the grain to the grain boundary gives time needed for void formation. Calculations of this time assuming an electrical potential gradient along the metal line gave estimates consistent with experimental observations. The increased values of migration energy under compressive stresses suggest that void formation could be deterred by applying such stresses.

The energy histogram method, introduced by Ferrenberg and Swendsen [Phys. Rev. Lett., 61, 2635, (1988) and 63, 1195, (1989)], was applied for the first time to the constant temperature molecular dynamics (MD) simulation of a two-dimensional (2D) system with incommensurate structures. We performed MD simulations for the stage-2 graphite intercalation compounds (GIC's) with Rb or K being the intercalants (Rb-GIC's and K- GIC's). The temperature dependence of the specific heat, Cv, is calculated for various sizes up to 864 atoms. The melting temperature was found to be 158 K for Rb-GIC's and 119 K for K-GIC's, respectively, which are in agreement with the experimental observations.

The diffusion transport of metal impurities in semiconductors is considered. Critical magnitude of dislocation density and critical sample size are obtained at which the diffusion kinetics change from the interstitial impurity regime of diffusion transport to the self-interstitial regime or the vacancy regime. It is predicted that the critical density of dislocations decreases with increasing temperature.

Diffusion of gold atoms into silicon crystals was considered. It was shown that W-shaped gold profiles in silicon can be explained in the framework of both the dissociative and the kickout mechanisms.