Comparative Study of Carbon Force Fields for the Simulation of Carbon Onions

We evaluate the performance of ten common carbon force fields for the interaction energies in double and triple layered carbon onions. In particular, we consider the C₂₀@C₆₀, C₂₀@C₈₀, C₂₀@C₁₈₀, C₈₀@C₂₄₀, C₆₀@C₂₄₀ and C₂₄₀@C₅₄₀ double-layer carbon onions and C₆₀@C₂₄₀@C₅₄₀ and C₈₀@C₂₄₀@C₅₄₀ triple-layered carbon onions. We consider the following carbon force fields: Tersoff, REBO-II, AIREBO, AIREBO-M, screened versions of Tersoff and REBO-II, LCBOP-I, 2015 and 2020 versions of ReaxFF, and the machine-learning GAP force field. We show that the ReaxFF force fields give the best performance for the interaction energies of the cabon onions relative to density functional theory interaction energies obtained at the PBE0-D3/def2-TZVP level of theory. We proceed to use the ReaxFF-15 force field to explore the interaction energies in a giant ten-layered carbon onion with a C₆₀ core and show that the interaction energy between the outer layer and the inner layers increases linearly with the number of layers in the carbon onion (with a squared correlation coefficient of R² = 0.9996). This linear increase in the stabilization energy with each consecutive layer may have important thermodynamic consequences for describing the formation and growth of large carbon onions.