Project M2 is devoted to the development and application of approaches to elucidate and predict excited  state phenomena in aggregates of varying numbers of atoms and molecules. This includes studying model systems throughout the molecular precursor-to-material pathway, from individual atoms and molecules to aggregates and extended systems such as two-dimensional (2D) materials on different length-scales (e.g., monolayers or multilayers), assembled molecules (e.g., dimers or monolayers), and molecules adsorbed or covalently bonded on surfaces. The primary focus will be on transition metal dichalcogenides, V-VI chalcogenides, and perovskites for materials, and photoswitches for molecular systems. To address this challenge, we will use a combination of ab initio quantum chemical methods, such as time-dependent density functional theory and many-body perturbation theory, along with data science techniques. This approach will help us to explore structure- and size-dependent properties of excited state phenomena, including electronic absorption and emission spectra and the yield and rate of certain photoinduced processes. The developed procedures will be critically validated through close collaboration with experimental spectroscopic projects, reinforcing our understanding of how certain structures determine the excited state properties of materials.