Nano Science and Technology Institute
Nanotech 2003 Vol. 2
Nanotech 2003 Vol. 2
Technical Proceedings of the 2003 Nanotechnology Conference and Trade Show, Volume 2
Chapter 12: Computational Methods and Numerics

Evolving Molecular Force Field Parameters for Si and Ge

Authors:A. Globus, E. Ricks, M. Menon and D. Srivastava
Affilation:CSC at NASA Ames, US
Pages:516 - 519
Keywords:force field, parameterization, genetic algorithm, molecular dynamics
Abstract:A genetic algorithm (GA) has been developed to fit parameters for multi-species reactive inter-atomic force field functions. While GA has successfully parameterized force fields in the past [Hunger, Beyreuther, Huttner, Allinger, Radelof, Zsolnai (1998), Hunger, Huttner (1999), Cundari, Fu (2000)], until now GA has not been applied to parameterization of reactive force fields suitable for critical nanotechnology tasks. Given an analytic form (of which several are available), fitting parameters to multi-species reactive force fields is extremely tedious and error prone because the parameter space is large and includes complex correlations. As a result, parameters are available for only a few reactive systems (Si, C, and a few others). By automating parameter fitting, we seek to significantly expand the reactive systems that may be investigated using molecular dynamics. The ability to model reactive solid systems with fast molecular dynamics, as opposed to much more compute-intensive quantum calculations, will enable a wide variety of crack propagation, thin-film deposition and etching, ion and cluster bombardment, surface diffusion and reactions, molecular machine manufacture, nanotubes strength and dynamics, and many other studies of critical importance for the development of nanotechnology. Our method, involving both near equilibrium and far from equilibrium configurations in the fitting procedure, is unlikely to get trapped in any local minima and can be extended to incorporate direct experimental measures as well. As a proof of concept, we demonstrate the procedure for the Stillinger-Weber (S-W) potential [Stillinger, Weber (1985)] by (a) reproducing the published parameters for Si by using S-W energetics in the fitness function, (b) evolving a new set of parameters, with a fitness function based on a non-orthogonal tight-binding method [Menon, M., Subbaswamy, K. R. (1993)] better suited to Si cluster energetics than the published S-W potential, and (c) evolving parameters for Ge clusters, for which S-W parameters were previously unavailable. Evolution was driven by a fitness function based on the energies and forces calculated for Sin and Gen clusters (n
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