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. This project is a continuation of our previous SNIC projects on the topic, which have resulted in several publications (most recently ACS Photonics https://doi.org/10.1021/acsphotonics.2c00066, NanoLetters https://doi.org/10.1021/acs.nanolett.2c02327). Additionally, during the past two years this project has supported the publication of one PhD (https://research.chalmers.se/publication/523675) and one Licentiate thesis (https://research.chalmers.se/publication/529167). The requested computational resources would enable the continuation of the research on this topic.