Force Field Benchmarking for Molecular Dynamics Simulations of Polyethylene Melt

• Panachok Kaewrahan, Chemical Engineering, University of Delaware

• Dionisios Vlachos, Chemical and Biomolecular Engineering, University of Delaware
• Rajas Mehendale, Chemical and Biomolecular Engineering, University of Delaware

Molecular dynamics (MD) simulations are a powerful tool for studying the properties of polymer melts at the molecular level. In classical MD simulations, force fields determine the interactions between atoms, including bond stretching, angle bending, torsional interactions, and nonbonded interactions. Different force fields use various functional forms and types of interactions to appropriately represent different systems.

This research aims to systematically test and benchmark polymer properties using both all-atom and coarse-grained force fields for varying chain sizes of polyethylene (PE) melts. Benchmarking these force fields is crucial for understanding the accuracy of the properties derived from the simulations, which helps in selecting the most suitable force field for specific applications. We also wish to compare the trade-offs between coarse-grained and all-atom force fields, since coarse-grained force fields sacrifice atomistic accuracy but allow for longer timesteps and fewer particles in the simulation, allowing the systems to be simulated to longer timescales, optimizing the computational resources.

The force fields evaluated include OPLS-AA, OPLS-UA, OPLS/2020, TraPPE, and AMBER.  Linear alkanes of lengths C₂₀, C₇₀, and C₁₄₂ were used as PE surrogates, and simulated in LAMMPS at melt densities adopted from experiments. The chain characteristics analyzed were the distributions of the radius of gyration, end-to-end distances, mean internal distances, and dihedral angles. The different force fields used in the simulations were compared using these distributions that provide insights into the size and the conformations adopted by these chains.

Understanding the accuracy of the force fields and the conditions or properties which they represent accurately is crucial to aiding in choosing the appropriate force field for the required applications. As an extension to this research, we aim to simulate and benchmark these PE chains on a catalyst surface, focusing on their adsorption and conformations during depolymerization. This future work will be a primary focus of my senior thesis.