Strong light-matter interactions in both the single-excitation and coupled regimes are attracting significant attention due to their potential use in emerging quantum and nonlinear optics applications, as well as by the possibilities for modifying material-related properties in strongly-coupled systems. An accurate theoretical description of such systems at the intersection of quantum optics and quantum chemistry is challenging, and typically requires large computational resources. In the project we will study light-matter interactions between metallic nanoparticles composed of single or multiple metals and nearby molecules or semiconducting clusters. By studying the how the collective plasmon resonances interact with the adsorbates we will elucidate nanoscale coupling in these systems and how this interaction leads to modified material-related properties such as spectral and spatial hot carrier generation, or modification of adsorbate excitations. In this project we will utilize time-dependent density-functional theory to (1) gain first-principles insight into plasmonic processes in metal nanoparticles and systems exhibiting qualitatively similar collective electronic excitations and strong light-matter coupling at the nanoscale with adsorbates and to (2) develop multicomponent systems and materials for future applications. These calculations will also serve as input of optical properties of individual nanoscale entities into (3) multiscale calculations where they will interact with each other within Fabry Perot or other complex optical cavities. Interactions will be modeled at the retarded electromagnetic level, including Casimir interactions, which are crucial for self assembly of nanoscale objects. This project is a continuation of our previous NAISS projects on the topic, which have resulted in several publications (most recently Physical Review Letters https://doi.org/10.1103/f4kv-pk93, Journal of Physical Chemistry Letters https://doi.org/10.1021/acs.jpclett.4c03517, Small Structures https://doi.org/10.1002/sstr.202400409, npj 2D Matter. Appl. https://doi.org/10.1038/s41699-025-00524-w). Additionally, during the past year this project has supported one PhD thesis (https://research.chalmers.se/en/publication/548675). The requested computational resources would enable the continuation of the research on this topic.