NIST Materials-focused Empirical Potentials Repositories and Efforts

Summary

Because classical empirical potentials/force-fields are widely used to model material behavior via Molecular Dynamics or Monte Carlo simulations, NIST recognized the need for easy-to-use repositories that provide the users with the force fields themselves in a downloadable format, as well as with the systematic evaluations of material properties computed using such interatomic potentials. Our repositories cover both hard and soft matter representing various functional forms and types of bonding. A quick description of each repository is provided below together with the link to their home pages, where more details and documentation can be found.  These complementary efforts allow users to find information they need to make decisions about whether to use the models under consideration.

Description

SOFT MATTER

WebFF: Force-field repository for organic and soft materials

Frederick R. Phelan Jr.,¹ and Huai Sun^2
1Materials Science and Engineering Division, NIST, Gaithersburg, MD 20899
²Aeon Technology Inc. and School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, China

WebFF is an open and extensible force-field (FF) repository, designed to support the Materials Genome Initiative (MGI) for organic and related soft materials. The repository is built using the NIST Materials Data Curation System (MDCS) which supports ontology based database descriptions using XML schema. Here are some of the main features of WebFF:

• Users interact with the repository through two main portals. The Data Curation Portal supports upload of published force-field data with appropriate metadata descriptors to support provenance based data sharing. New datasets may be curated interactively or using a python based toolset to upload large datasets en masse. Data curation requires an authorized account. 
• The Data Exploration Portal supports search for force-field data based on the curated metadata descriptors and download in a number for common formats. We will work with the user community to expand output coverage. 
• The initial release of the repository will feature two integrated XML schemas. The first schema supports Class I organic force-fields in such as OPLS, Amber and CHARMM style representations. The seconds supports Class II style force-fields such as CFF, PCFF, COMPASS and TEAMFF. 
• Schemas and tools are being developed to expand the initial repository to cover other classes of soft matter force-field data, specifically, for united atom (UA) and coarse-grained models (CG). The latter is in conjunction with our related soft matter project, the COMSOFT Workbench, whose principle function is progressive force-field (FF) parametrization.
• Descriptive website: https://www.nist.gov/programs-projects/webff-force-field-repository-organic-and-soft-materials
• Repository website (coming soon): http://webff.nist.gov/ 

HARD MATTER

Interatomic Potentials Repository Project (IPR)

Lucas Hale, Zachary Trautt, Francesca Tavazza, Chandler Becker
Materials Science and Engineering Division, NIST, Gaithersburg, MD 20899

The Interatomic Potentials Repository Project provides tools and resources for helping users of classical interatomic potentials perform meaningful simulations and calculations.

• The Interatomic Potential Repository website (https://www.ctcms.nist.gov/potentials) serves as host to the parameter files of over 150 interatomic potentials. There is no restriction on the format of the hosted potentials, and all potential records are verified by the original authors to be consistent with their published versions.  These potentials are widely downloaded and used in other projects, both inside and outside of NIST.
• A listing of known external resources supportive of classical atomistics is also provided on the website for researchers to easily find and discover. This includes other potentials repositories, simulation codes, analysis tools, and reference databases.  The focus is on tools and information to support appropriate selection of models and calculation methods.
• Selected materials properties for the hosted potentials are being computed and displayed, allowing for users to compare the different models and intelligently select the best for their study of interest.    This is focused primarily on properties such as dislocation monopoles, stacking fault energy surfaces, and complex vacancy structures.  The scripts and algorithms are made available for users.
• The atomman Python package (https://github.com/usnistgov/atomman) provides a simple, yet powerful, representation of atomic systems in Python. The focus of atomman is in performing and analyzing complex, large-scale atomistic simulations containing crystalline defects. It has built-in tools allowing for it to be a wrapper around the LAMMPS MD simulation software, as well as converters to other known Python atomistic representations. 
• The iprPy computational framework (https://github.com/usnistgov/iprPy) is the open source collection of calculation scripts and tools used to obtain the potential-dependent materials properties listed on the website. An emphasis is placed on designing the calculations to be easily sharable, have transparent and teachable methodology, have the capability of being performed in a high-throughput manner, and to produce structured and complete results records.

JARVIS-FF

Kamal Choudhary, Francesca Tavazza
Materials Science and Engineering Division, NIST, Gaithersburg, MD 20899

The JARVIS-FF database (http://www.ctcms.nist.gov/~knc6/periodic.html) provides a user-friendly web interface for comparing basic properties of metallic and ceramic materials to density functional theory (DFT) and experimental reference data.

• JARVIS-FF contains a systematic computation of bulk and un-reconstructed surface energetics, elastic properties, point-defect energies and phonon calculations for a large variety of metallic and ceramic materials, using all the empirical potentials that are publicly available through LAMMPS and the NIST Interatomic Potentials Repository.
• The JARVIS-FF database provides all the input files needed to reproduce the data shown, as well as direct links to corresponding density functional theory (DFT) and experimental data, where available, to give the user an easy way to evaluate the applicability of each interatomic potential to the specific property under examination.
• The data are presented in a user-friendly web interface to enable further material design and discovery. For instance, for each element or compound chosen, the interface shows at a glance the corresponding elastic constants computed with all the potentials available for such a material, so that the users can easily judge which force field best suits their needs.
• JARVIS-FF provides the user with the ability to compare the thermodynamic stability of materials simulated using force-fields to DFT stability data. Specifically, using the tools provided at https://github.com/JARVIS-Unifies/JARVIS-FF, the user can compute the classical convex hull for the material under examination using the interatomic potential of choice, as well as compare it with the corresponding DFT convex hull obtained from energetic data from the Materials Project (https://www.materialsproject.org/)