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

Abstract

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.