Local Molecular Properties of Non-Interacting and Hydrogen-Bonded Water Systems in Vibrational Strong Coupling Regimes

In polaritonic chemistry, the cavity-modification of local molecular properties under collective vibrational strong coupling within optical cavities plays a pivotal role in understanding and controlling chemical reactivity. While significant theoretical advancements have been made in this area, the majority of existing studies rely on model-based approaches and focus on molecular ensembles in the dilute gas limit. However, extending these studies to more complex systems, such as condensed phases or larger molecular aggregates, is not only essential but also presents a significant challenge due to the increased complexity and the demand for more accurate and computationally intensive methods. Here, we present an investigation of cavity-modified local molecular properties in both non-interacting water molecular ensembles and hydrogen-bonded water clusters, utilizing an atomistic, fully ab initio and analytical approach, which combines the cavity Born--Oppenheimer density functional theory with the Hirshfeld partitioning scheme to quantify local dipole moment and polarizability. We observe that the cavity induces non-zero changes in the local dipole moment and polarizability, with the dipole self-energy term being critical for these modifications, particularly in configurations where the total dipole moment of the molecular ensemble is zero. In hydrogen-bonded water clusters, hydrogen bonding enhances cavity-induced polarization in acceptor molecules while suppressing it in donors, and it mitigates the cavity-induced reduction in local polarizability. These results underscore the interplay between intermolecular interactions and cavity effects on molecular properties in optical cavities.