On-the-Fly Cavity-Molecular Dynamics of Vibrational Polaritons

TL;DR

This work addresses the challenge of simulating vibrational strong coupling and vibro-polaritons in large molecular systems by developing an on-the-fly MD framework that propagates nuclear motion in real space while treating the cavity field in reciprocal space using a beyond-long-wavelength light–matter Hamiltonian. The approach combines DFTB-SCC with a hub-and-spoke parallelization, enabling simulations with thousands of atoms and many cavity modes, and introduces the open-source CavOTF package. A key finding is that time-dependent Born charges are essential for accurate dynamical simulations, whereas Mulliken charges can reproduce qualitative linear spectra but cause spurious heating, limiting their use for dynamics or energy transport studies. The work demonstrates angle-resolved vibro-polariton spectra for liquid water and provides a practical, scalable tool for in silico exploration of cavity-mediated control of chemistry and energy flow in realistic materials.

Abstract

In this work, we combine the density functional tight-binding (DFTB) approach with a light-matter Hamiltonian beyond the long-wavelength approximation to propagate the dynamics of vibrational polaritons formed by coupling molecular vibrations to confined radiation inside a Fabry-Pérot optical cavity. Here, we develop a parallelized propagation scheme with lightweight inter-CPU communication by exploiting the sparse nature of the light-matter interactions in the real space representation. We find that the computationally expensive Born charges required for our propagation can be replaced with the computationally inexpensive Mulliken charges to obtain qualitatively accurate linear spectra especially when the nonlinearity (arising from molecular vibrations) of the light-matter interaction term is not substantial. However, the same approach may not be suitable to be used for studying cavity modification of energy transport or chemical dynamics as this approximation leads to spurious heating of the light-matter hybrid system. We demonstrate the utility of this on-the-fly approach to compute angle resolved polaritonic spectra of water. We implement our approach as an open-source computational package, CavOTF, which is available on GitHub.

Figures (5)

• Figure 1: Schematic illustration of light-matter interactions and the real-reciprocal used here for propagating vibro-polariton dynamics. Schematic illustration of light-matter interaction (a) in the standard multi-mode dipole gauge description and (b) in real space. (c) Schematic illustration of the algorithm used here and implemented in our computational package CavOTF.cavOTF
• Figure 2: (a) Born charges distribution of Oxygen and Hydrogen atoms in liquid water and (b) their time-dependent fluctuations.
• Figure 3: Schematic figure of water (a) outside and (b) inside an optical cavity. Simulated molecular spectra spectra of H-O-H bending motion (c) outside and (d) inside of the cavity under vibrational strong coupling. The effects of using Mulliken vs Born charges on (e) temperature and (f) cavity modified molecular spectra.
• Figure 4: Cavity modified molecular spectra and angle resolved photonic spectra under vibrational strong coupling. Panels (a)–(d) show cavity modified molecular spectra at various light–matter coupling strengths for $\eta_0 = 0.0001$ with $\omega_0 = 0.19\:\mathrm{eV}$, and panels (f)–(i) show the corresponding spectra for $\eta_0 = 0.00034$ with $\omega_0 = 0.43\:\mathrm{eV}$. Angle resolved photonic spectra are shown in panel (e) for $\omega_0 = 0.19~\text{eV}$ and in panel (j) for $\omega_0 = 0.43~\text{eV}$.
• Figure 5: Angle-resolved photonic spectra under vibrational strong coupling when using Mulliken charges vs Born charges at (a) $\omega_0 = 0.16$ eV, $\omega_0 = 0.19$ eV, $\omega_0 = 0.22$ eV, and $\omega_0 = 0.43$ eV. Here we use the light-matter coupling $\eta = \eta_c \cdot \omega_0^{3/2}$ with $\eta_c = 0.00105$.