The Interatomic Potentials Repository (IPR) at the National Institute of Standards and Technology (NIST) provides a valuable service to the molecular simulation community. The NIST IPR accepts interatomic potential parameter files contributed by developers and provides access to these files and accompanying information on their website. Links are also provided to results obtained using these potentials from computations performed by NIST IPR staff.
The OpenKIM project collaborates with NIST by regularly exchanging information. Many of the potentials archived in OpenKIM were originally submitted to the NIST IPR. Users uploading parameterized models (parameter files) to OpenKIM have the option of requesting that the content also be sent to the NIST IPR.
Despite the similarities, OpenKIM is fundamentally different from the NIST IPR. OpenKIM not only stores parameter files and completed calculations, but is also a computational infrastructure that is integrated with major simulation codes that support the KIM Application Programming Interface (API) standard. Potentials archived in OpenKIM consist of functioning computer code that can be used seamlessly with these programs. In addition, the potentials are verified and tested within the OpenKIM system using an automated framework called the KIM Processing Pipeline.
Specific OpenKIM capabilities that differ from the NIST IPR include:
OpenKIM not only archives parameter files, but also includes the computer implementation of the potential (interatomic model) using these parameters. Since implementation details can completely change simulation results, archiving only parameters is insufficient for reproducing simulation results. See details here: M. Wen et al., "Interpolation Effects in Tabulated Interatomic Potentials", MSMSE, 23, 074008 (2015).
Interatomic potentials archived in OpenKIM (called "KIM Models") conform to the KIM API (application programming interface) that allows them to be used seamlessly with many major simulation codes including ASE, DL_POLY, GULP, LAMMPS, QC. See the list of codes supporting the OpenKIM standard. KIM provides a utility to easily download and install potentials on a user's resources.
Each potential in OpenKIM has a unique 12-digit identifier + 3-digit version number (called a "KIM ID") that can be cited in publications. Any changes to content are recorded for full provenance control. Changes that can affect simulation results are recorded as a version change. Citing the KIM ID makes it possible to access the exact potential (implementation+parameters) used in a simulation and reproduce the results.
Each potential has its own "model page" consolidating all the information on the model. This includes metadata provided by the developer (title, description, references in BibTeX format, guidelines on usage), verification checks and test results as explained below, and a wiki where the developer can provide additional information or documentation, and registered KIM users can add comments and feedback. A developer contributing a model to OpenKIM can directly edit the content on this page making changes as necessary.
Each potential uploaded to OpenKIM is automatically subjected to a unique set of "verification checks" that verify the correctness and coding integrity of the potential (e.g. numerical derivative check on forces, smoothness, invariance with respect to rigid-body motions, memory leaks, thread safety, and so on). These results are displayed in a dashboard on the potential's model page.
Each potential uploaded to OpenKIM is automatically subjected to a host of material property calculations (called "KIM Tests"), such as equilibrium crystal structures, elastic constants, defect formation and migration energies, surface energies, phonon spectra, … Users can develop their own tests which when uploaded to OpenKIM are run against all applicable potentials archived in the system. Test results are displayed on a potential's model page both numerically and using "Visualizers" (which can also be developed by Users).
All content in OpenKIM (test results, in particular) are accessible via web queries. This means a user can programmatically access this information. For example, in the input script for a molecular dynamics simulation of a crystalline material, the equilibrium lattice constant(s) predicted by the potential being used can be retrieved from openkim.org. For more on queries, see the OpenKIM Query page.
The OpenKIM project and the NIST IPR continue to collaborate and cooperate. OpenKIM imports contents from the NIST IPR and the NIST IPR imports content from OpenKIM. For content uploaded to both OpenKIM and the NIST IPR, each archive provides a hyperlink to the corresponding content archived in the other. It is the intention of the OpenKIM project to continue working closely with the NIST IPR to serve the needs of the materials research community.