Preface

The Born–Oppenheimer adiabatic approximation represents one of the cornerstones of molecular physics and chemistry. The concept of adiabatic potential-energy surfaces, defined by the Born–Oppenheimer approximation, is fundamental to our understanding of molecular spectroscopy and chemical reaction dynamics. Many chemical processes can be rationalized in terms of the dynamics of the atomic nuclei on a single Born– Oppenheimer potential-energy surface. Nonadiabatic processes, that is, chemical processes which involve nuclear dynamics on at least two coupled potential-energy surfaces and thus cannot be rationalized within the Born– Oppenheimer approximation, are nevertheless ubiquitous in chemistry, most notably in photochemistry and photobiology. Typical phenomena associated with a violation of the Born–Oppenheimer approximation are the radiationless relaxation of excited electronic states, charge-transfer processes, photoinduced unimolecular decay and isomerization processes of polyatomic molecules.