“…In polaritonic chemistry, depending on whether the quantized cavity modes are coupled via their characteristic frequencies to electronic or vibrational degrees of freedom of molecules, the situation is described as electronic-strong coupling (ESC) or vibrational-strong coupling (VSC), respectively. Under ESC, it becomes possible to modify the photochemistry/photophysics of molecules including charge transfer processes and electronic spectroscopy, and photoinduced reactions can be influenced. − Similarly, for VSC, the vibrational spectra of molecules are altered by the formation of light–matter hybrid states, and even the chemical reactivity of the ground state can be modified. ,− ,− The observed effects of molecular ESC and VSC are often discussed phenomenologically, and understanding of the underlying microscopic and macroscopic physical mechanisms, especially with respect to the effects of VSC, is still incomplete. ,− In our recent work we have shown numerically that the interaction between an optical cavity and ensembles of molecules not only leads to cavity detuning and a change of the optical length but also allows for a local molecular polarization mechanism under strong collective vibrational coupling in the thermodynamic limit. The interplay of microscopic and macroscopic polarization is due to cavity-mediated dipole–dipole interaction.…”