Quantum Chemical Approaches to Treat Mixed‐Valence Systems Realistically for Delocalized and Localized Situations

The state of the art in the quantum chemical treatment of mixed-valence systems near the borderline between class II and class III character is discussed. Starting from my personal history of entering the field, initially regarding organic MV systems, later transition-metal-based complexes, more recent developments, which were made since a comprehensive 2014 review, are discussed. This includes use, by many groups, of a pragmatic computational protocol based on the BLYP35 global hybrid functional combined with continuum solvent models and various modifications of the protocol. A wide range of questions regarding organic MV systems, up to and including aspects of electron transport, have been addressed. Recent work on the importance of conformational aspects in organometallic molecular wires and other applications to organometallic systems are also reviewed. More recent improvements of computational protocols based on gas-phase benchmarking and local hybrid functionals are touched upon, as are more advanced treatments of solvent effects. Dynamical aspects of solute and solvent on electron transfer and spectroscopy are not covered in this chapter.