ChIMES: A Force Matched Potential with Explicit Three-Body Interactions for Molten Carbon

Abstract

We present a new force field and development scheme for atomistic simulations of materials under extreme conditions. These models, which explicitly include two- and three-body interactions, are generated by fitting linear combinations of Chebyshev polynomials through force matching to trajectories from Kohn-Sham density functional theory (DFT). We apply our method to liquid carbon near the diamond/graphite/liquid triple point and at higher densities and temperatures, where metallization and many-body effects may be substantial. We show that explicit inclusion of three-body interaction terms allows our model to yield improved descriptions of both dynamic and structural properties over previous empirical potential efforts, while exhibiting transferability to nearby state points. The simplicity of our functional form and subsequent efficiency of parameter determination allow for extension of DFT to experimental time and length scales while retaining most of its accuracy.

Ultrafast shock synthesis of nanocarbon from a liquid precursor

“…MD simulations can supply an atomistically resolved view of this process but are challenging since relevant scales span over three orders of magnitude: from the molecular scale, where bond formation and breaking events occur, to the condensed domains scale, where formation and coarsening dynamics determine the system evolution. To overcome this, we have developed a reactive force field capable of first-principles accuracy for systems under extreme conditions 35,36 (including liquid carbon 35 ), which is computationally efficient and enables multi-million atom simulations that can reach nanosecond timescales (see "Methods" section for further details).…”

“…In summary, the nanocarbon synthesis sequence that emerges from the simulations appears to be oxygen-rich polymeric seeds formation, followed by densification and oxygen elimination, and particle growth via evaporationcondensation. At late times the nanodroplets are pure molten carbon 35,40 with an oxygen-decorated surface, occupying ≈10% of the volume (a typical cluster is described in Supplementary Note 4 and shown in Supplementary Fig. 7).…”

“…Model for Efficient Simulation (ChIMES) 35,36 was employed for simulations in the present work. ChIMES is a generalized many-body classical reactive force-field constructed from linear combinations of Chebyshev polynomials, machine learned to Density Functional Theory (DFT) 45 molecular dynamics simulations.…”

“…ChIMES is a generalized many-body classical reactive force-field constructed from linear combinations of Chebyshev polynomials, machine learned to Density Functional Theory (DFT) 45 molecular dynamics simulations. The substantial flexibility afforded through the use of a polynomial basis enables ChIMES to serve as a high-fidelity proxy for DFT while providing a four-orders-ofmagnitude increase in simulation efficiency and linear system size scalability 35,36 . The ChIMES CO model used herein contains explicit two-and three-body interactions and has been shown to yield good agreement with the DFT-calculated structure, dynamics, and energetics at the conditions studied in this work 35 .…”

Development of the ChIMES Force Field for Reactive Molecular Systems: Carbon Monoxide at Extreme Conditions

“…Recently, the Chebyshev Interaction Model for Efficient Simulation (ChIMES) was developed which, in its current state, describes explicit two-31 and three-body interactions 32 through linear combinations of Chebyshev polynomials. ChIMES models are parameterized by force matching 33 to short DFT MD trajectories, circumventing the issue of scant experimental training data at high T and p. To date, this model has been proven highly suitable for extending DFT forces to molten metallic carbon at extreme conditions 32 , as well as liquid water under moderate T and p 34 . Here, we apply ChIMES to a problem of higher complexity, namely that of reactive carbon condensation in carbon monoxide under extreme conditions.…”