We calculated frequency-dependent first hyperpolarizabilities for electro-optic Pockels effect, β(ω; 0, ω), in π-conjugated molecules containing azomethine bonds using the CNDO/S-CI sum-over-states method. First, we examined the dependence of static β on formulas for the two-center Coulomb integral, γAB. The choice of the γAB formula has little effect on the relative β value, but a considerable effect on the absolute value. Then we investigated the relationship between the magnitude of β(ω; 0, ω) and the donor/acceptor substitution sites. We found that the molecule with no donors or acceptors can be divided into donor and acceptor regions whose existence causes the donor/acceptor site-dependence of β(ω; 0, ω).

Angular dependence of structure sensitive magnetic properties such as coercivity and permeability has never been adequately explained by a quantitative physical model or theory. The present study investigates the angular dependence of magnetic properties of materials with different microstructures. Measurements were taken under compressive and tensile uniaxial stress. The materials investigated were plain carbon steels with compositions of 0.1 % -0.8% carbon. Alloy steels with compositions of 2.25 % Cr and 1.0 % Mo were also investigated. The effects of uniaxial stress on permeability along the direction of the stress can be described by including an effective field Hσ given by:

Experimental results are compared to model using Hσ.

The normal modes of vibration and their frequencies are calculated for benitoite, a mineral whose crystal structure (space group D23h) consists of three-membered silicate rings (Si3O9) linked by Ba+2 and Ti+4 ions. The calculation unambiguously assigns the normal modes to the lines in the Raman and infrared spectra and determines the atomic interactions. On the assumption that mode mixings and splittings due to inter-ring interactions are small, the normal frequencies of the isolated ring of C3h, symmetry are determined. The identification of normal modes characteristic of three-membered silicate rings may prove to be a valuable guide in the interpretation of the infrared and Raman spectra of amorphous silicates, potentially leading to new information on the ring statistics of these materials.

Surface defects both focus the local electric field and enhance its intensity in the vicinity of the defect. Using a finite element model of an inhomogeneous dielectric film, we have examined the relationship between the defect shape and local electric field in the film and defect. We find that raised defect regions that are peaked have the largest local fields while the slope of removed regions has little effect on the field in raised defects. Both peaked and flat surface defects can lead to large enhancements of the local electric field above that predicted by effective medium approximation (EMA) methods.

We present the results of a detailed molecular-dynamics investigation of anharmonic effects at the (100) and (110) surfaces of Ni, with the interactions between atoms described using embedded-atom potentials. We find both surfaces to suffer an anomalously large thermal expansion, with a concomitant rapid increase of the mean-square amplitudes of vibration. For the (100) surface, our results for the surface expansion can be directly compared to the low-energy electron diffraction (LEED) measurements of Cao and Conrad;1 the agreement is excellent. The anomalies appear at a temperature of about 900 K for the (100) surface, well below the onset of disordering, thereby confirming that the observed anomalies indeed arise from anharmonicities. Our simulated LEED intensities are found to reproduce well the observed intensities. The phonon spectra in the anharmonic regime will be discussed, and a detailed comparison between the two surfaces will be established.

The order-disorder transformation in CU3Au has been investigated using a combined Molecular Dynamics and Cluster Variation Method approach. Free energy minimization has been performed using the Natural Iteration technique. The calculated temperature dependence of enthalpy, lattice dynamics, short-range and long-range order parameters have been successfully compared with experimental data.

Based on realistic many-body potentials molecular-dynamics simulations are carried out for PdH0.03 and NbH0.02. The H motion is investigated at two different temperatures, T = 300 and 600K, paying attention to the vibrational and diffusive motion. We find that the motion of H in Nb, a bcc metal, is more complicated than in Pd, a fee metal, and the differences are discussed. When detailed comparison is made with quasielastic neutron scattering data for H in Pd at 600K, we argue that in order to characterize the diffusion correctly, one has to include nonadiabatic effects.

The molecular dynamics method was used to simulate the plastic deformation of small copper single crystals. Dislocations were created near the tips of cracks. A very sharp yield stress was found.

We present the results of Monte Carlo and Molecular Dynamics simulations of some thermodynamical properties of Cu3Au. The calculations rely on an empirical n-body potential that reproduces satisfactorily the critical temperature, Tc, the temperature dependence of the lattice constant and atomic vibrational amplitudes as well as Cowley's short-range order parameters above Tc. Our results show that relaxation effects decrease considerably the formation energy of antisite defects and therefore should explicitly be considered for a realistic description of the transition.

The evolution of a random and initially homogeneous distribution of parallel and infinitely extended edge dislocations is studied by using elastic energy minimization without and in presence of a periodic external stress, τa. During the energy minimization without external stress (relaxation), randomly distributed dislocation dipoles are formed whereas, when the external stress is acting, the dislocations condense in walls. We investigated the spatial periodicity of this microstructure, λ, as a function of, τa, and of the total dislocation density. The elastic energy of the stress-induced microstructure is found to be comparable to the value obtained by relaxation. Thereby, emphasis is given to the dynamical character of patterning. A phenomenological model has been developed, explaining the correlation between λ and τa found in the simulations and comparing favorably with existing experimental data.

By using a Van de Graaff accelerator we have implanted 2 MeV protons and deuterons into InP samples. These implanted samples were annealed in 50 °C increments in the 200 to 600 °C temperature range for 30 minutes each and then measured by the infrared absorption spectroscopy. A group of four different local vibrational mode absorption lines is observed, each of which arises from the hydrogen-phosphorus stretching vibration with different defects or impurities at nearest-neighbor sites. Each mode exhibits an annealing behavior which is different than any of the other absorption lines. The annealing results are shown and discussed in terms of Green's function calculations of the defects responsible for the two absorption lines.

An embedded-atom-method (EAM) interatomic potential [1] for bcc-iron is derived. It is fitted exactly to the lattice parameter, elastic constants, an approximation to the unrelaxed vacancy formation energy, and Rose's expression for the cohesive energy [2]. Formation energies and relaxation volumes of point defects are calculated. We find that the relative energies of the defect configurations depend on the functional fitting details of the potential considered, mainly its range: the experimental interstitial configuration of lowest energy can be reproduced by changing this parameter. This result is confirmed by calculating the same defect energies using other EAM potentials, based on the ones developed by Harrison et al. [3].

We study the structure and growth sequence of small palladium clusters Pdn using the Many-Body Alloy (MBA) potential and simulated annealing techniques. Our results show the preference of compact polyhedral structures. These equilibrium structures are compared with the bulk Pd crystal in terms of cohesive energies and nearest neighbor distances. Both the cohesive energy and the nearest neighbor distance show a slow convergence to bulk behaviors. By analyzing the detailed structures and cohesive energies, we find that Pd4, Pd7 and Pd13 are magic number structures, which are the consequence of their high symmetry and large coordination number.

The geometries, the linear (α) and third-order (γ) polarizabilities of several bipolaron model compounds and cyanines are computed using the semi-empirical AM1 parameterization. The effects of aromaticity, charge localization and bond length alternation are considered.

These proceedings summarize recent work in our group studying the structure of interfaces involving molecular fluids. Two types of systems are discussed. First, we summarize simulations of the structure and surface tension of liquid-vapor interfaces of the alkanes eicosane and decane. Then, we describe the results of simulations of the confined films studied in surface force apparatus and atomic force microscopy experiments. Our simulations show that in both films of normal and branched alkanes, the formation of a layered structure is observed. The branching inhibits this layering, especially in the narrowest pores. In addition an examination of the molecular motions indicates that a transition to a solid or glassy state is not a prerequisite for layering or oscillating solvation forces.

AM 1 semiempirical calculations on model structures of neutral, anionic and cationic poly(p-phenylenebenzobisthiazole), PBZT, offer an explanation why the PBZT anion has been shown to be an electrical conductor while the PBZT cation is not. To wit, the highest occupied molecular orbital (HOMO) in the PBZT model structure is centered primarily on the sulfur atoms, resulting in concentration of the positive charge on the sulfur atom when the cation is formed. The lowest occupied molecular orbital (LUMO) is far more delocalized and consequently anion formation produces a species which has electronic charge delocalized over the entire model structure. Similar calculations on model structures of neutral, anionic and cationic poly(p-phenyienebenzobisoxazole), PBO, indicate that both frontier orbitals, i. e., the HOMO and the LUMO are delocalized.

Complimentary molecular dynamics and Metropolis Monte Carlo algorithms for the atomistic simulation of crystals at constant temperature and homogeneous tensorial pressure are summarized. The novel aspect of computational physics which unites these methods is the extension of the virial theorem to nonlinear elastic media. This guarantees the dynamical balance between the internal pressure, as determined by the interatomic potential, and effective external pressure, as determined by the applied laboratory pressure, and includes the elastic response of the material. Numerical examples are presented.

We study nanoindentation of silicon using nonequilibrium molecular dynamics simulations with up to a million particles. Both crystalline and amorphous silicon samples are considered. We use computational diffraction patterns as a diagnostic tool for detecting phase transitions resulting from structural changes. Simulations of crystalline samples show a transition to the amorphous phase in a region a few atomic layers thick surrounding the lateral faces of the indentor, as has been suggested by experimental results. Our simulation results provide estimates for the yield strength (nanohardness) of silicon for a range of temperatures.

We present experimental data which show that large amplitude coherent lattice vibrations can be impulsively excited using short optical pulses of light. Furthermore, using a quasi-equilibrium model, we show how the these coherent lattice vibrations suggest a ~ 7 THz modulation of the semiconductor-to-metal transition in Ti2O3.

A simple dynamical Ising model on a fixed lattice with a single bond energy parameter has been used to simulate the kinetics of diffusion during solid-state phase transformations in binary metallic alloys. The results of these simulations are compared with direct real-space measurements of the atomic distributions of elements in alloys, obtained with the position-sensitive atom probe. Despite the simplicity of the model, there is good quantitative agreement between the development of micro-structure in the simulation and the nucleation and growth of cobalt-rich precipitates in copper-cobalt alloys.