ITR: Simulating Extended Time and Length Scales using Parallel Kinetic Monte Carlo and Parallel Accelerated Dynamics

University Of Toledo, Toledo OH

Investigators

Abstract

This award is the result of a proposal submitted to the Information Technology Research (ITR) initiative. The research will be done in collaboration with Los Alamos National Laboratory. A long-standing obstacle to the understanding of condensed phase systems is that many important processes occur on a time-scale that is not easily accessible with 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, relevant processes that are activated, i.e., infrequent events, often take place on a time-scale of microseconds or even longer. 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. Recently, a variety of accelerated dynamics techniques, including hyperdynamics, parallel replica dynamics, and temperature-accelerated dynamics have been proposed in order to speed up the simulation of infrequent events in molecular dynamics. In particular, temperature-accelerated dynamics has been quite successful in extending the time-scales for simulations since it allows realistic simulations oflow-temperature processes over time-scales as long as seconds and even hours. However, due to the fact that the computational work required scales as the square or even as the cube of the number of atoms N, this technique can only be applied to extremely small systems. As a result, realistic simulations of materials over both extended time and extended length scales have not been possible. The research here will use parallel computations in order to extend accelerated dynamics so that both large system sizes and long time-scales can be simulated simultaneously. The development of such a capability to simulate both mesoscopic systems sizes and long time-scales should present a major breakthrough in our ability to carry out realistic atomic simulations. In parallel with this effort, sparse-computational algorithms will be developed in order to reduce the exponent corresponding to the dependence of the computational work on the system size in accelerated dynamics. The development of such algorithms is based on the realization that eliminating non-local moves or groups of non-local moves from the force calculations involved in these methods during the search for saddle-points should significantly reduce the dependence of the computational work required on the cluster size N. Parallel replica dynamics will also be used in both techniques to extend the time-scales of the simulations. As a first specific application of spatially parallel accelerated dynamics we will focus on simulations of metal-on-metal epitaxial growth at low temperature using temperature-accelerated dynamics. This is a problem of great interest due to the observation of a variety of unexplained phenomena including nanoscale facetting and strain-induced mound regularization at low tempertures. The methods developed here should be applicable to a much broader range of systems as well. %%% This award is the result of a proposal submitted to the Information Technology Research (ITR) initiative. The research will be done in collaboration with Los Alamos National Laboratory. A long-standing obstacle to the understanding of condensed phase systems is that many important processes occur on a time-scale that is not easily accessible with 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, relevant processes that are activated, i.e., infrequent events, often take place on a time-scale of microseconds or even longer. 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. Recently, a variety of accelerated dynamics techniques, including hyperdynamics, parallel replica dynamics, and temperature-accelerated dynamics have been proposed in order to speed up the simulation of infrequent events in molecular dynamics. In particular, temperature-accelerated dynamics has been quite successful in extending the time-scales for simulations since it allows realistic simulations oflow-temperature processes over time-scales as long as seconds and even hours. However, due to the fact that the computational work required scales as the square or even as the cube of the number of atoms N, this technique can only be applied to extremely small systems. As a result, realistic simulations of materials over both extended time and extended length scales have not been possible. The research here will use parallel computations in order to extend accelerated dynamics so that both large system sizes and long time-scales can be simulated simultaneously. The development of such a capability to simulate both mesoscopic systems sizes and long time-scales should present a major breakthrough in our ability to carry out realistic atomic simulations. In parallel with this effort, sparse-computational algorithms will be developed in order to reduce the exponent corresponding to the dependence of the computational work on the system size in accelerated dynamics. The development of such algorithms is based on the realization that eliminating non-local moves or groups of non-local moves from the force calculations involved in these methods during the search for saddle-points should significantly reduce the dependence of the computational work required on the cluster size N. Parallel replica dynamics will also be used in both techniques to extend the time-scales of the simulations. As a first specific application of spatially parallel accelerated dynamics we will focus on simulations of metal-on-metal epitaxial growth at low temperature using temperature-accelerated dynamics. This is a problem of great interest due to the observation of a variety of unexplained phenomena including nanoscale facetting and strain-induced mound regularization at low tempertures. The methods developed here should be applicable to a much broader range of systems as well. ***