Simulating Nonequilibrium Processes over Extended Time- and Length-scales using Parallel Accelerated Dynamics

University Of Toledo, Toledo OH

Investigators

Abstract

TECHNICAL SUMMARY This award supports computational and theoretical research and education on a long-standing obstacle to the understanding of condensed-phase systems: many important processes occur on a time-scale that is not easily accessible with conventional methods. In order to address this gap, a variety of accelerated dynamics techniques including hyperdynamics, parallel replica dynamics, and temperature-accelerated dynamics have been proposed. In particular, temperature-accelerated dynamics has been quite successful in extending the time-scales for simulations since it allows realistic simulations of low temperature processes over timescales as long as seconds and even hours. However, due to the fact that the computational work required for serial temperature-accelerated dynamics scales as the number of atoms, N, cubed, this technique can only be applied to extremely small systems. In order to address this problem, the PI has recently developed a method for parallel temperature-accelerated dynamics simulations or "parTAD," which is based on spatial decomposition combined with the PI's synchronous sublattice algorithm, which scales as log(N). Using this method, the PI has studied the low-temperature growth of Cu/Cu (100) over extended length-scales in order to explain recent observations of vacancy formation and compressive strain. The PI aims to use parTAD to carry out parallel temperature-accelerated dynamics simulations of a variety of non-equilibrium processes over extended time- and length-scales. In addition, the PI plans to develop mew methods which will enable the temperature-accelerated dynamics method to study processes at higher temperatures as well as to study 3D systems and systems with long-range interactions. These include the development of a method to locally adapt the high temperature parameter which controls the "boost" in the accelerated-dynamics simulations in each processor to optimize the efficiency for a given configuration, as well as to deal with the "low-barrier" problem. To extend the possible size of events that can be handled by simulations and also further enhance the efficiency of parTAD simulations, the PI also aims to develop a hybrid approach to parTAD in which spatial decomposition is coupled with medium scale parallel molecular dynamics simulations and localized saddle-point searches. Using these methods the PI will carry out parallel accelerated dynamics simulations to understand important non-equilibrium processes which cannot be easily studied with lattice-based methods, including: (1) Crystalline-to-amorphous transition in low-temperature semiconductor growth (2) Early stages of growth of amorphous Si on SiO2 substrates (3) Ordering, intermixing and defect formation in submonolayer and multilayer Fe/Cu(100), Cu/Ni(100), and Co/Cu(111) growth (4) Radiation damage and defect mobility in MgO The work will provide opportunities for educational and outreach activities with broad national, international, and societal impact. Some of the methods and results developed from this project will also be incorporated in a joint undergraduate-graduate course on Computational Physics taught by the PI at the University of Toledo along with a graduate course on Thin-Films and Surface Physics in which students will learn about new theoretical advances and state-of-the-art implementation and empirical evaluation techniques. New algorithms that are developed contribute to the cyberinfrastructure of the broader materials research community. NON-TECHNICAL SUMMARY This award supports computational research and education to develop and apply algorithms and software to simulate processes on time scales that are important to the underlying science but out of the range of conventional simulation methods. For example, molecular dynamics is generally limited to nanoseconds because of the small time-step required for the integration of the equations of motion. However, important infrequent events often take place on a time scale of microseconds, seconds, or even hours. Examples include the evolution of the surface morphology during crystal or film growth, the diffusion of point defects in solids, and the migration of grain boundaries during plastic strain. The PI aims to build on research performed under previous NSF funding to develop new simulation tools that can access longer time scales, higher temperatures, and larger systems. With the PI's existing and enhanced tools he will tackle specific materials problems involving how semiconductor materials grow and the morphology of new layers, how growing materials layers can intermix with the material upon which they are grown, and how imperfections in the arrangement of atoms near surfaces move and their affect on the growth process. The PI will also apply the new methods to materials growth and the dynamics of materials after irradiation. These studies will be carried out in close connection with specific experiments and will enhance our understanding of materials growth. The work will provide opportunities for educational and outreach activities with broad national, international, and societal impact. Some of the methods and results developed from this project will also be incorporated in a joint undergraduate-graduate course on Computational Physics taught by the PI at the University of Toledo along with a graduate course on Thin-Films and Surface Physics in which students will learn about new theoretical advances and state-of-the-art implementation and empirical evaluation techniques. New algorithms that are developed contribute to the cyberinfrastructure of the broader materials research community.