Two-molecule theory of polyethylene liquids.

This article focuses on the implementation and evaluation of the two-molecule (TM) theory, a microscopic self-consistent field approach with a hypernetted-chain (HNC)-like solvation potential, applied to polyethylene (PE) melts. TM theory models the liquid structure by averaging over configurations of two polymer chains in an effective field representing the surrounding molecules, offering a formally derived closure to the reference interaction site model (RISM) equation. Compared to the widely used polymer-RISM (PRISM) theory with the Percus–Yevick (PY) closure, TM theory provides improved predictions of local and long-range liquid structure and better agreement with many-chain molecular dynamics (MD) simulations, particularly for liquids with repulsive Lennard-Jones interactions. While TM theory underestimates the pressure and overestimates compressibility for liquids with full Lennard-Jones potentials due to the strength of the HNC solvation potential, it remains computationally much faster than MD simulations and shows promise for extension to more complex polymeric and multicomponent liquids.