Atomistic Potentials and the Future of Nanomaterials Metrology

We will provide researchers and designers with a database of evaluated interatomic potentials for metals and alloys; thermodynamic, kinetic, and mechanical materials properties generated for each potential; and experimental or first-principles reference data for comparison. The use of atomistic simulations will thus be facilitated as a tool to predict properties of materials in systems, such as nanomaterials, where direct measurement is time-consuming or extremely difficult.

We are meeting with researchers in industry, government, and academia to determine their highest priorities. These meetings are conducted through conference presentations and workshops, but also through informal discussions with colleagues. In response to these interactions, we created the NIST Interatomic Potentials Repository as the central location for distribution of interatomic interaction models of materials, including metals and alloys. We have also responded to user requests for conversion tools between software formats, and we are defining a standard set of tests and the computational infrastructure necessary to evaluate new and existing potentials as required to verify or complement other measurements. Additionally, we are compiling a database of available experimental measurements, first-principles calculations, and reported properties from atomistic potentials. Our public database of properties will ultimately allow users to compare interatomic potentials and experimental results.

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.

 

Effective pair potentials show Aluminum interactions

To build and disseminate a database of interatomic potentials, evaluations, and reference data, we have begun 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.

To this end, in April 2008 we hosted the inaugural 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, including accuracy, standardization, and evaluation methods. Participants in this workshop included representatives from Ford Motor Company, GE, General Motors, United Technologies Research Center, Materials Design, NIST, various national laboratories (NIST, Sandia, Air Force Research Lab, Ames, Los Alamos), and universities. The workshop planned for 2009 will further address these topics, but it will also include a greater focus on the standardization of interatomic potential formats and discussions about the role of experimental measurements in fitting and validating interatomic potentials.

 

Local atomic structure in the melt from pair correlation functions.

Among the most requested items during these interactions 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 are working on both the conversions and tools to be released publicly.

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. 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 are also examining formats and methods for publishing or disseminating the collected information in a user-friendly way.

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.

Volumes for liquid Aluminum.