Variable time domain discretization methodology for molecular dynamics simulation of metallic compounds

Jamal Zeinalov

doi:10.32920/ryerson.14656692

Abstract

The present work proposes a methodology to improve the computational requirements of molecular dynamics simulations while maintaining or improving the fidelity of the obtained results. The most common method of molecular dynamics simulation at present is the multi-force, constant time-step, explicit computation, which advances a single time step at a time to determine the next state of the system. The present work proposes a variable time-step strategy, where a single large simulation is subdivided into multiple time domains which redistribute computational resources where they are needed the most: in areas of higher than average potential or kinetic energy or highly dynamic areas around impurity clusters, void formations and crack propagations. The research focuses on the simulation of metallic compounds, as these form the basis of most common molecular dynamics simulations, and have been very thoroughly investigated over the years, thus providing a very extensive body of work for the purpose of comparison and validation of the proposed methodology. The novel methodology presented in this work allows to alleviate some of the limitations associated with the molecular dynamics methodologies and go beyond traditional scales of simulation. The proposed method has been observed to deliver 5 to 20 percent increase in simulation size domain while maintaining or improving the accuracy and computational cycle time. The benefits were observed to be greater for large simulations with one or more areas of higher than average kinetic or potential energy levels, such as those found during crack initiation and propagation, coating-substrate interface, localized pressure application or large thermal gradient. The large difference allows for very clear prioritization of computational resources for high energy areas and as a result provides for faster and more accurate simulation even with increased domain size. Conversely, this method has been observed to provide little to no benefit when simulating stable systems that are undergoing very slow change, such as (relatively) slow change in ambient temperature or pressure, or otherwise homogeneous internal and external boundary conditions. However, for the majority of applications described above, including coating deposition and additive manufacturing, the proposed methodology will yield substantial increase in both simulation size and accuracy, since in the aforementioned processes kinetic and potential energy gradients across the simulation are typically very significant

Highlights

Molecular Dynamics simulations have become increasingly more important in the investigation of material properties and interactions at nanometer to micrometer scale over the past several decades
The molecular dynamics simulations are used to predict increasingly complex processes of material treatment and manufacturing, including coating deposition on turbine blades and new material development including structural composite and additive manufactured parts. These new manufacturing processes rely on addition of relatively small amounts of material to build up a complete part or coating level and are difficult to simulate using traditional continuum mechanics methods, because of the increased importance of molecular structure on such small scales. As such the methods employed in the molecular dynamics analysis are becoming increasingly more important to study within the field of aerospace engineering to ensure the comparable level of fidelity enjoyed by the simulations that are employed at more common scales using continuum mechanics models
The most significant limitation of the molecular dynamics simulations is the sheer number of atoms required for the simulation to predict the most important physical characteristics of the materials, including thermal expansion and conductivity, structural properties, strength, modulus of elasticity, isotropic and anisotropic behaviour, as well as failure modes and propagation rates

Summary

Introduction

Molecular Dynamics simulations have become increasingly more important in the investigation of material properties and interactions at nanometer to micrometer scale over the past several decades. The molecular dynamics simulations are used to predict increasingly complex processes of material treatment and manufacturing, including coating deposition on turbine blades (and other high temperature, corrosive applications) and new material development including structural composite and additive manufactured parts. These new manufacturing processes rely on addition of relatively small (under micron) amounts of material to build up a complete part or coating level and are difficult to simulate using traditional continuum mechanics methods, because of the increased importance of molecular structure on such small scales. Larger models tend to better predict these properties but come at an increased computational cost

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