From Ground to Excited: Modern Strategies for Modeling Light–Matter Interactions

Abstract: Understanding and predicting how molecules and materials interact with light is central to modern computational chemistry, underpinning research in photochemistry, photovoltaics, spectroscopy, and biology. Yet, accurately describing electronically excited states remains a formidable task due to the challenges of simulating electronic structure, nuclear dynamics, and relativistic effects using high-level theoretical methods. In this lecture, I will survey the conceptual and practical challenges in simulating electronically excited states and discuss how different theoretical frameworks ranging from single-reference excited-state approaches to multireference methods address these issues. I will then focus on recent methodological developments from our group that enable accurate yet efficient treatment of complex excited-state phenomena in molecules and materials. Examples will illustrate how these new tools can be used to predict excitation energies, analyze photochemical pathways, and interpret experimental spectra. Finally, I will outline current frontiers and open questions in excited-state theory, highlighting opportunities for innovation at the interface of electronic structure theory, nonadiabatic dynamics, machine learning, and high-performance computing. The goal of this talk is to provide both conceptual insight and a practical perspective for students and researchers interested in exploring the rapidly evolving field of excited-state simulations. Keywords: Electronic structure theory, excited states, multireference methods, nonadiabatic dynamics, spectroscopy