Field Heterogeneity Flips Chemistry: Mesoscopic Electromagnetic Boundaries Reshape Microscopic Solvation via FDTD-MD Multiphysics Coupling

Abstract Simulating electromagnetic field effects on soft matter typically relies on the assumption of spatially uniform driving fields. While valid for isolated molecules, this approximation fails in mesoscopic systems—such as aqueous aerosols or plasmonic hotspots—where dielectric boundaries create steep macroscopic field gradients. Here, we present a multiscale computational framework that couples Finite Difference Time Domain (FDTD) electrodynamics (micron-scale) with Molecular Dynamics (MD) (nanoscale) to resolve field heterogeneity with unprecedented rigor. Key Findings Photonic Jet Formation: We resolve the spatial focusing of Terahertz radiation (f = 2.30 THz) in a 25 μm water droplet, revealing a localized "photonic jet" with 4.0× intensity enhancement at the shadow-side interface. Thermodynamic Inversion: Coupling this position-dependent field to the MD integrator reveals a thermodynamic chiral solvation bias of ΔΔG ≈ -3.2 kJ/mol for L-alanine solvation. This contrasts sharply with the baseline uniform-field prediction (ΔΔG ≈ +0.8 kJ/mol). Statistical Significance: The shift is statistically significant (p < 0.01) and represents a fundamental "flipping" of the chiral preference due entirely to mesoscopic geometry. Validation: The framework is validated by energy conservation tests ( λ/2) where multiphysics coupling is mandatory. It establishes the theoretical basis for Resonant Bi-Harmonic Rectification as a driver of prebiotic homochirality and provides the computational proof for the "Baird Frequency" mechanism in aerosol chemistry. Methodology Simulations utilized a custom FDTD-MD coupling patch for GROMACS, integrating 4D vector field maps generated via MEEP/Lumerical solvers into the force calculation loop.