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The elastic moduli of a solid can be determined from the thermal fluctuations of the stress. The so-called stress-fluctuation approach is useful in that it can provide insights into the molecular origin of a particular mechanical response, and it leads to faster convergence than methods based on fluctuations of the strain. Unfortunately the implementation of the stress-fluctuation approach is more demanding than that of the strain-fluctuation approach, particularly for atomistic models involving intramolecular interactions (e.g., bending and torsion). In this study a simple numerical method is proposed to evaluate the elastic moduli of atomistic models from knowledge of atomistic forces. It is shown that this approach leads to fast and reliable prediction of the elastic moduli for two different classes of materials. In one example the elastic moduli of crystalline silicon are compared to those reported in the literature. In the other example the elastic moduli of an atomistic polymer model for poly(methyl methacrylate) are shown to be in good agreement with experimental data. more...

The high-temperature behavior of a high-angle twist grain boundary, a free surface, and planar arrays of voids of various sizes, all on the (001) plane in copper, are studied through molecular-dynamics simulation using an embedded-atom-method potential. Independently, we determine the thermodynamic melting point, {ital T}{sub {ital m}} of this potential through an analysis of the free energies of a perfect crystal and the liquid phase. It is found that an ideal crystal consisting of nearly 1000 atoms may be superheated over 200 K above {ital T}{sub {ital m}} while the introduction of any of the defects listed above nucleates melting at any temperature above {ital T}{sub {ital m}}. We conclude that nucleation of the liquid phase at extrinsic defects is the most rapid, and therefore the dominant, mechanism of melting. more...

A new method for the molecular-dynamics simulation of bulk planar interfaces at high temperatures is presented. The method uses the basic Parrinello-Rahman (constant-stress) scheme, modified for the application to inhomogeneous systems. Since our computational cell contains only one interface with two-dimensional (2D) periodic border conditions, we are able to study isolated interfaces all the way up to melting. The interaction between boundaries which may lead to their annihilation at higher temperatures, which is a problem when 3D periodic borders are applied, is thus avoided. As an application, the method is used to study the stability of a grain boundary at high temperatures. Observations on a possible connection between grain-boundary migration and ''premelting'' are discussed. more...

The high-temperature behavior of both a high-angle twist grain boundary and a free surface on the (110) plane of silicon are investigated using molecular dynamics and the Stillinger-Weber potential. It is found that, above the thermodynamic melting point, melting is nucleated at the grain boundary or surface and propagates through the system with a velocity that increases with temperature. We conclude that, due to the relatively fast nucleation times, melting in real crystals should be initiated at grain boundaries and surfaces, a conclusion that is entirely in accord with experiment. more...