Accuracy of Classical Potentials for Polyethylene Structures Away from Equilibrium

Realistic simulations of polymers require a large number of atoms due to the anisotropic nature of their morphology. Although density functional theory (DFT) is considered highly accurate, it is too computationally expensive to handle large systems. Classical potentials used for molecular dynamics studies are trained on experimental or quantum mechanical equilibrium structures, and their accuracy away from equilibrium is not well known. This work is a comparative study of two widely used classical potentials—Optimized Potentials for Liquid Simulations (OPLS) and the reactive force-field (ReaxFF)—with respect to DFT. Their performance for polyethylene (PE), a simple model polymer, is benchmarked by comparing the classical energies, forces, and stresses against DFT. Additionally, a pressure-temperature phase diagram for PE is computed using OPLS and ReaxFF. Although the classical potentials do not perfectly capture the DFT and experimental behavior, the results are close enough to justify their use when large system sizes are required. Consequently, these results will be used for shock simulations involving far from equilibrium structures.