This project focuses on light-matter interactions revealed by collective plasmon resonances and molecular excitations, and in particular, their strong coupling.Strong light-matter interactions in both in 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. In this project we will utilize time-dependent density-functional theory to (1) gain first-principles insight in plasmonic processes and strong light-matter coupling on the nanoscale and to (2) develop optimal systems and materials for future applications. We also plan to extend the scale of interactions between plasmonic particles into the retarded regime of van der Waals forces, namely Casimir interactions, which are crucial for self assembly of nanoscale objects. This project is a continuation of our previous SNIC projects on the topic, which have resulted in several publications (most recently ACS Nano https://pubs.acs.org/doi/10.1021/acsnano.3c11418, NanoLetters https://doi.org/10.1021/acs.nanolett.4c03153, Science Advances https://doi.org/10.1126/sciadv.adn1825, npj 2D Matter. Appl. https://doi.org/10.1038/s41699-025-00524-w). Additionally, during the past two years this project has supported the publication of two Licentiate theses (https://research.chalmers.se/en/publication/535761, https://research.chalmers.se/publication/540525). The requested computational resources would enable the continuation of the research on this topic.