Frequently Asked Questions

The purpose of this project is to provide a repository of interatomic potentials for atomistic simulations (e.g. molecular dynamics) with comparison tools and reference experimental and ab-initio data in order to facilitate the evaluation of these potentials for particular applications. Our goal is not to judge that any particular potential is "the best" because the best interatomic potential may depend on the problem being considered. For example, some interatomic potentials which have been fit only to the properties of solid phases may model solid surface properties better than one fit with solid and liquid properties. However, the second potential will probably better represent properties which have a strong liquid contribution (e.g. crystal-melt interfacial properties). Additionally, we are not limiting the repository to a single class of material (e.g. metals), interatomic potential format (e.g. Embedded-Atom Method), or software package. As we obtain interatomic potentials for other materials or in other formats, we will include them with proper website modifications.

Disclaimer: Due to the range of formats, we are not able to guarantee that the interatomic potentials will work with particular software packages. Rather, we provide the interatomic potentials with appropriate citation information, and it is up to the user to determine whether a potential will work with given software and produce the expected results.

LAMMPS formats: Most of the hosted parameter files are in a format compatible with the LAMMPS software. See the LAMMPS documentation for more details about the specific LAMMPS pair_style formats listed with the implementations. Note that if the format listed in the repository includes (custom) or (modified), then the potential may require building a modified version of LAMMPS with additional or modified code.

OpenKIM models: The repository provides cross-links to many of the interatomic potentials hosted by OpenKIM. Note that the OpenKIM models differ in that they are designed to be installed using the kim-api and do not necessarily have separate parameter files associated with them. See the OpenKIM documentation for more details about how to setup and use it.

Other formats: Additionally, a variety of other formats are also hosted here. A (non-comprehensive) list is as follows

• IMD option EAM are EAM parameter files for use with the IMD code
• EAM setfl are parameter files in the setfl format, but are not directly compatible with the associated LAMMPS pair styles.
• SPaSM are EAM parameter file tabulations for use with the SPaSM MD software.
• Moldy FS are EAM parameter files for use with the Moldy MD software.
• MoldyPSI combines a parameter file with Fortran90 routines to calculate the EAM functions and their derivatives.
• Dynamo MEAM are MEAM parameter files in the format used by LAMMPS, but requires a modification of the LAMMPS code to work properly.
• MEAM parameters are EAM parameter file tabulations for use with the SPaSM MD software.
• MEAM splines are MEAM spline parameter files that are not directly compatible with the associated LAMMPS pair style.
• GULP is a parameter file for use with the GULP MD software.
• EAM tabulated functions and Finnis-Sinclair tables are tabulations of the EAM potential functions not in the setft format.
• ADP tabulated functions are tabulations of the ADP potential functions not in the setft format.
• FORTRAN is a FORTRAN script that can be used to generate parameter files.
• ASE calculator is an implementation that can be directly used with the ASE python code as a calculator.
• deepmd-kit is the implementation and code of an ML potential in the deepmd-kit format.
• GAP are Gaussian approximation potentials. These are in the format used by the QUIP software, which can be interfaced with simulation software such as LAMMPS.
• Equations is simply a document that lists the potential's functional form

Differences in the plots of the graphical form of the potentials and what is reported in the associated publications can be due to a variety of reasons:

• File formatting and units. Different developers and software packages use different data and file formats to present their interatomic potentials. As such, parameter files that implement the same potential into different simulation codes can substantially differ in what parameters and function tabulations are included, and what units those values need to be in. For this reason, it is very important to always check the documentation of the software the potential will be used with to verify it is correctly implemented.
• Alternate equivalent parameterizations. The functional forms of interatomic potentials are often complex and involve parameters and/or functions that are not independent of each other. As such, there may be multiple ways of parameterizing a potential in a given format that results in the final functional form being mathematically equivalent. One well known example of this is that the EAM potential equation allows for two different invariant transformations to be performed that alter how the pair, embedding, and electron density functions look but result in the same total energy for elemental interactions. While these transformations do not alter elemental interactions, they do play a part in the cross-elemental interactions that multi-element alloy potentials include. The issue of the EAM invariant transformations is discussed in several places, such as Y. Mishin, "Interatomic potentials for metals," in Handbook of Materials Modeling, edited by S. Yip (Springer, Dordrect, The Netherlands, 2005), Chap. 2.2, pp. 459-478. and A.E. Carlsson, "Beyond pair potentials in elemental transition metals and semiconductors," in Solid State Physics, Volume 43, Ehrenreich and Turnbull (eds.) (1990).
• Publication typos. It is an unfortunate fact that the articles associated with interatomic potentials often contain typos that miss report information about the interatomic potentials. Some examples are errors in the potential's functional form equations, incorrect values in the table of parameters, incorrect reported property values, or even reported values and parameters being for different parameterizations of the same functional form. Such issues often result in published potentials being unreproducible from the associated articles alone. For this reason, the Interatomic Potentials Repository primarily prefers to host parameter files obtained from the original creators/authors. When first-party implementations are not available, we do our best to verify that any alternate implementations reproduce the reported results in the publication.

The homepage for LAMMPS is https://www.lammps.org/. The LAMMPS code can also be found on github at https://github.com/lammps/lammps.

Please send an email to potentials@nist.gov which includes the publication information. This will be used to create a list of papers citing the website in order to help us better understand how the website is being used

1. Properly cite any interatomic potentials you used with the associated references found with each listing.
2. The Interatomic Potentials Repository itself can be cited with
   • C.A. Becker, F. Tavazza, Z.T. Trautt, and R.A. Buarque de Macedoc (2013), "Considerations for choosing and using force fields and interatomic potentials in materials science and engineering," Current Opinion in Solid State and Materials Science, 17, 277-283. DOI: 10.1016/j.cossms.2013.10.001
   Website URL: https://www.ctcms.nist.gov/potentialsWebsite DOI: 10.18434/m37
3. If you use atomman, iprPy or any of the resulted property predictions, please also cite
   • L.M. Hale, Z.T. Trautt, and C.A. Becker (2018), "Evaluating variability with atomistic simulations: the effect of potential and calculation methodology on the modeling of lattice and elastic constants," Modelling and Simulation in Materials Science and Engineering, 26, 055003. DOI: 10.1088/1361-651X/aabc05

Send the potential to potentials@nist.gov with format and citation information, and optionally any other notes you wish to include about what the potential was designed for, or important information about the implementation. A prototype page will be created that will be posted publicly upon the approval of the submitter.