Chemical Dynamics and Bond-Order Potentials

Extract

Tremendous advances in computing speed, increasingly clever algorithms, and more accurate interatomic-force models have made atomistic simulation a powerful tool in many areas of materials science and engineering. Indeed, from traditional areas such as the study of grain boundaries, fracture, and point defects to somewhat less traditional areas such as nanometer-scale engineering and device fabrication, atomistic simulations are providing exciting new data and insights that cannot be obtained in any other way. Central to the success of an atomistic simulation is the use of an appropriate force model. For simulations requiring large systems and/or long times, the computational efficiency offered by classical potential-energy functions is necessary. However, obtaining quantitative results requires a model that can both accurately describe an appropriate database of physical properties and that is transferable to structures and dynamics beyond those to which it is fit. The latter property is especially critical if an atomistic simulation is to have useful predictive capabilities. While an extensive and well-chosen database from which parameters are determined is important, transferability ultimately depends on the chosen functional form. The definitive mathematical form, however, has yet to be developed.

This article covers my attempts to develop classical potential-energy functions based on an empirical bond-order formalism that put qualitative and semiquantitative features of chemistry into large-scale condensed-phase simulations. To help explain this work, the next section gives a brief tutorial of the “chemistry” of few-body potentialenergy surfaces. Although familiar to chemists, the concepts discussed in this section may not be as familiar to mate rials scientists and engineers. A discussion of the bond-order formalism and how it can be related to both solid-state structure and chemical dynamics follows. The article ends with a discussion of two applications of this formalism, modeling chemical dynamics near a shock front and the reactive chemistry of diamond deposition.