Sequential FDTD-MD field transfer for mesoscopic dielectric heterogeneity and reduced Langevin structural-bias modelling.

Standard molecular dynamics (MD) protocols simulate external electromagnetic fields under the assumption of spatial uniformity. While mathematically valid for isolated nanoscale ensembles, this uniform-field approximation breaks down in mesoscopic systems-such as aqueous aerosols, plasmonic hotspots, and complex heterogeneous dielectrics-where structural boundaries generate steep macroscopic field gradients. In this work, we present a sequential multiscale framework in which finite-difference time-domain (FDTD) electrodynamics resolves mesoscopic field localization and transfers a time-averaged local rectified field to atomistic molecular dynamics (MD) simulations. Solving Maxwell's equations for a 25 µm aqueous droplet under bi-harmonic terahertz irradiation (f1 = 2.3 THz, f2 = 4.6 THz) reveals the formation of a photonic jet, yielding a 4.0× localized field intensity enhancement at the shadow-side boundary. The time-averaged rectified field is sampled at the shadow-side boundary and transferred as an effective local forcing term. Atomistic MD simulations quantify the response of Ace-Ala-Nme in explicit TIP3P water, whereas a separately parameterized reduced Langevin model is used to explore the response of a chirality-sensitive improper-dihedral coordinate under idealized field-driven conditions. In the reduced Langevin model, the improper-dihedral reaction coordinate evolves from the initial reference state toward a long-lived state near 126° under the specified V3 forcing protocol. This result should be interpreted as a model-based prediction rather than a direct atomistic simulation of covalent chiral inversion. This sequential framework establishes a rigorous quantitative protocol for modelling non-equilibrium, field-driven physical chemistry, serving as a versatile methodology for analyzing multiscale electrodynamic transport in highly heterogeneous dielectric environments.