Atomistic Potentials and the Future of Nanomaterials Metrology

Summary:

This project provides a resource to address some of the challenges to the wider use of quantitative classical atomistic simulations (e.g. molecular dynamics or Monte-Carlo). This is done through the development of the NIST Interatomic Potentials Repository; workshops on atomistic simulations for industrial needs; thermodynamic, structural, and kinetic property evaluations for various interatomic potentials with comparison to available experiments; and new work on automated evaluations and standardization with KIM, the Knowledgebase of Interatomic Models (PI's: Ellad Tadmor, Jim Sethna, and Ryan Elliot).

Description:

Atomistic simulations are increasingly being used as a tool to predict properties of materials in systems, such as nanomaterials, where direct measurement is time-consuming or extremely difficult. The success of atomistic simulations depends critically on the fidelity of a specific model of interatomic interactions to the area of application. Interatomic potentials in metallic systems are typically derived and optimized against a small number of validating experiments, and they necessarily work best for specific materials and phases. Thus, these potentials may accurately model one system and range of parameter space but be inappropriate for another. It is therefore essential that engineers and researchers know which interatomic potentials are available and how well they reproduce material properties of interest.

To build and disseminate a database of interatomic potentials, evaluations, and reference data, we began by soliciting feedback from researchers in industry, government, and academia to ascertain which properties are most relevant for industrial design processes. Such properties include, but are not limited to, molar volumes, phase stability, microstructural information, thermodynamics of interfaces and surfaces, diffusion, and melting temperatures. Additionally, it is important to know how these quantities change with decreasing size, especially as nanoscale materials become increasingly important. This feedback has been incorporated throughout the project, and is reflected in the activities described below.

Major Accomplishments:

Among the most requested items was the development of a trusted location for the dissemination of interaction models. The NIST Interatomic Potentials Repository (www.ctcms.nist.gov/potentials/) answers this need by posting files of known origin with full citation information and notes about the formatting and use of the files in molecular simulation software. Various element and alloy interatomic potentials are available from multiple developers, and the number continues to grow. Users and developers have also requested conversions of interatomic interaction models between different file formats, and we include some potentials in multiple formats to reflect this.

Additionally, in 2008 and 2009, we hosted the NIST/MSEL Workshop on Atomistic Simulations for Industrial Needs to facilitate interactions between researchers in industry, academia, and government on issues related to the development and use of interatomic interaction potentials. Such issues include accuracy, standardization, and evaluation methods. Participants in this workshop have included representatives from Ford Motor Company, GE, General Motors, Intel Corporation, Accelrys, United Technologies Research Center, Materials Design, NIST, various national laboratories (NIST, Sandia, Air Force Research Lab, Ames, Los Alamos), and universities. Planning is under way for the 2010 workshop.

We have also performed structural, thermodynamic, and kinetic analyses on several popular models of aluminum optimized to have good liquid properties but which display qualitatively different crystallization behavior in order to document these differences in a systematic way. In this we have examined the pair correlation functions, thermal expansions, and liquid diffusion coefficients, respectively, as functions of temperature and pressure. These results are compared with experimental information where possible. This has helped identify issues related to technical implementation of property evaluations in order to maintain consistency in approach (simulation times, data analysis methods, etc.). We find quantitative differences between the liquid properties, highlighting the importance of the chosen interatomic potential in atomistic simulations.

This work is the first step in a larger program to provide researchers with the tools to evaluate how well interatomic potentials for multicomponent alloys, as well as interactions of metals with ceramics and polymers, model the properties of real materials. These tools will help improve the quality of atomistic simulations for design and nano-metrology.

Effective Pair Potentials