Force Field Optimization and Surface-Induced Cooperative Nucleation in <i>n</i> -Octane Crystallization via Molecular Dynamics Simulations

Phase-change energy storage represents a pivotal research direction in the energy field, with its core lying in an atomistic understanding of the solid-liquid phase transitions. Here, we optimized the force field parameters for <i>n</i>-octane through two-dimensional scanning in the (σ, ε) space, with the optimized parameters then employed to investigate its melting point (<i>T</i>_m) and solid-liquid phase transition mechanisms. Preliminary simulations with the widely used PYS-type FFs underestimated the <i>T</i>_m of <i>n</i>-octane (C8) by ∼91 K. Finite-size analysis revealed convergence for a system of 681 chains. A comprehensive sensitivity study was carried out to quantify the influence of energy, size and torsion parameters. The resulting optimized parameter set collapses the <i>T</i>_m prediction error to merely 1 K. Subsequent crystallization runs performed at ∼16% undercooling reveals a surface-nucleation pathway, in contrast to the extended-chain nucleation assumed by the classical Lauritzen-Hoffman theory. Moreover, the in-plane orientation of chains within the crystal dictates the structural motif of the emerging surface nuclei. These findings provide a reference framework for accurate phase-transition modeling and offer molecular-level guidance for the design of organic phase-change materials.