Quantum-chemical embedding methods for treating local electronic excitations in complex chemical systems

International audience; Quantum chemistry has become an invaluable tool for studying the electronic excitation phenomena underlying many important chemical, biological, and technological processes. Here, we review quantum-chemical approaches for modeling such phenomena. In particular, embedding methods can be particularly useful for treating localized excitations in complex chemical systems. These split the total system into a number of interacting subsystems. The electronic excitations processes occurring in the subsystem of interest are then treated with high accuracy, while its environment is taken into account in a more approximate way. In this review, we use a formulation based on the formally exact frozen-density embedding theory as our starting point. This provides a common framework for discussing the different embedding approaches that are currently available. Moreover, it also forms the basis of emerging methods that allow for a seamless coupling of density-functional theory and wavefunction based approaches, both for ground and excited states. These provide new possibilities for studying electronic excitations in large systems with predictive quantum-chemical methods.