Real space electrostatics and non-equilibrium molecular dynamics for nanoscale transport

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J. Daniel Gezelter of the Uninversity of Notre Dame is supported by an award from the Chemical Theory, Models and Computational Methods (CTMC) program and the Computational and Data-Enabled Science and Engineering (CDS&E) program focused onunderstanding of metallic nanoparticles (small particles a few billionths of a meter in diameter) that are in contact with water and other solvents. Dr. Gezelter is developing computational methods and software to study heat transfer at the surfaces of the particles in the important case that they are protected by attached molecules. This topic is of significant interest in the areas of energy, materials research, and the health sciences, particularly for the particles that have been proposed for use in photothermal therapies where laser light is used to cause very localized heating. The details of the solvent structure and dynamics at this interface determine how energy flows away from the metal particles. In order to make these particles medically useful, it is important to determine the mechanism for the increase of the temperature in the surrounding environment following laser heating. This allows the computational methods to predict heat conductance at the surface, which can be difficult to determine experimentally. The goals of this proposal are to develop: (1) algorithms to efficiently model the transfer of energy across nanoparticle surfaces, and (2) methods to very efficiently model molecules like water that are polar, i.e., have positive and negative charges dominate at opposite ends. All of the algorithms and software supported by this project will be released to the public and other researchers under a permissive open source license. The PI organizes the OpenScience Project, a website which highlights and makes available examples of useful scientific tools and research codes as well as tools that can be used by a broader audience (scientifically-inclined but non-expert).Two significant method-development projects for molecular simulation are part of this work. This project expands efficient real-space electrostatic models to cover point-multipoles, and the real-space models are compared with established (but more expensive) simulation techniques. These models use two distinct approaches to truncated Taylor series approximations to the electrostatic kernel at a real-space cutoff radius. Coarse-grained models for proteins, lipids, and liquids are all expected to benefit from the efficiency gains of a real-space approach. The work also involves algorithmic development of non-equilibrium techniques to study interfacial properties like thermal conductance and interfacial friction at the curved interfaces of nanoparticles. These quantities are dynamic properties of interfaces, and are responses to non-equilibrium conditions or very rare fluctuations away from equilibrium behavior. Because of this, they are nearly impossible to measure using standard equilibrium molecular dynamics or Monte Carlo methods. To study interfacial transport, methods that create heat or angular momentum fluxes between materials are necessary. The combination of algorithmic development and application of these algorithms to realistic models for nanoparticles will provide insight into problems of intense scientific interest.