Abstract

Excited states are essential for understanding photochemical processes, ranging from photosynthesis and biological mechanisms of vision, to photocatalytic reactions and optoelectronics. However, accurate simulations of high-energy, charge transfer, or double excitations remain difficult with state-of-the-art techniques based on linear response theory. This talk will focus on an alternative “state-specific” perspective where excited states are represented by higher-energy variational stationary points of the electronic energy landscape. Firstly, I will describe the structure of the exact electronic energy landscape, the topological properties of exact excited states, and how approximate wave function ansätze form constrained subspaces. I will then characterise the different solutions that can be identified using a single Slater determinant or multiconfigurational wave function approximations in typical molecular systems. These examples highlight that state-specific excitations can provide compact representations of high-energy, charge transfer, and double excitations, while also demonstrating the challenges that must be addressed to achieve practical calculations in the future.