Multiscale coupled Maxwell's equations and polarizable molecular dynamics simulation based on charge response kernel model

Molecular Simulation Study of Surface-Enhanced Raman Scattering of Liquid Water

We developed in our previous study [J. Chem. Phys., 2021, 155, 174118, J. Phys. Chem. A, 2022, 126, 4762] a classical electronic and molecular dynamics simulation method to describe the optical response of metal material in solution based on an atomistic model by incorporating the classical equation of motion for free electrons under an applied electric field. To show further usefulness of the method, in the present study, we apply it to surface-enhanced Raman scattering of liquid water to examine the signal enhancement of the solution system caused by plasmon resonance effects of a silver nanoparticle.

Computational Analyses of Plasmonics of a Silver Nanoparticle in a Vacuum and in a Water Solution by Classical Electronic and Molecular Dynamics Simulations

We present basic optical responses of a silver nanoparticle (Ag309) in a vacuum and a water solution obtained by classical electronic and molecular dynamics (CEMD) calculations, where the CEMD is our previously developed force-field based molecular dynamics simulation method that incorporates the classical equation of motion for free electrons in metal and an interaction with the applied oscillating electric field (Yamada, A. J. Chem. Phys. 2021, 155, 174118)). The present work is the follow-up of the previous study. Calculated absorption spectra in the visible region with various conditions are reported together with parameter determination for realistic analyses as well as the verification of the method. Time-domain optical responses of Ag309 in water solvent are as well analyzed in terms of the plasmon resonance excitation under a subpicosecond light pulse and its thermal relaxation.

Classical electronic and molecular dynamics simulation for optical response of metal system

An extended molecular dynamics simulation that incorporates classical free electron dynamics in the framework of the force-field model has been developed to enable us to describe the optical response of metal materials under the visible light electric field. In the simulation, dynamical atomic point charges follow equations of motion of classical free electrons that include Coulomb interactions with the oscillating field and surrounding atomic sites and collision effects from nearby electrons and ions. This scheme allows us to simulate an interacting system of metals with molecules using an ordinary polarizable force-field and preserves energy conservation in the case without applying an external electric field. As the first applications, we show that the presented simulation accurately reproduces (i) the classical image potential in a metal-charge interaction system and (ii) the dielectric function of bulk metal. We also demonstrate (iii) calculations of absorption spectra of metal nano-particles with and without a water solvent at room temperature, showing reasonable red-shift by the solvent effect, and (iv) plasmon resonant excitation of the metal nano-particle in solution under the visible light pulse and succeeding energy relaxation of the absorbed light energy from electrons to atoms on the metal and to the water solvent. Our attempt thus opens the possibility to expand the force-field based molecular dynamics simulation to an alternative tool for optical-related fields.

Multiscale simulation of terahertz radiation process in benzimidazole crystal by impulsive stimulated Raman scattering

Comprehensive dynamics of coupled light wave and molecules in the terahertz wave generation process in an organic molecular crystal solid, 5,6-dichloro-2-methylbenzimidazole (DCMBI), induced by impulsive stimulated Raman scattering has been described by our previously developed multi-scale simulation, Maxwell + polarizable molecular dynamics method, where the propagation of macroscopic electromagnetic fields and microscopic molecular dynamics based on the force field model are numerically solved in the time domain. It has shown the behaviors of the excitation of Raman-active phonon modes by the irradiated pulse and terahertz radiation by molecular motions of infrared-active modes. Simulations of terahertz absorption and Raman spectroscopies of the DCMBI solid have also been performed to verify the applicability of the method to the terahertz optics. The calculated spectra are compared with the experimental measurements, showing good agreement. The detailed motions of the interacting electromagnetic fields and molecules occurred in the terahertz spectroscopies have also been provided, and the analyses have shown that rotational motions of the DCMBI molecules play key roles in the terahertz wave generation.