Non-adiabaticity: the importance of conical intersections

In this review, we discuss non-adiabatic processes focusing on the role conical intersections. Consequently after a brief historical perspective, we provide a thorough review of conical intersections in Section 2. Section 3 discusses the formulation of non-adiabatic dynamics in the adiabatic and diabatic representations. The effect of the geometric phase on near conical intersections dynamics is discussed. Section 4 discusses the construction of diabatic representations. Properties based diabatisations are discussed and the recently reported issue of diabolical singularities is described. A promising method which simultaneously fits and diabatises ab initio adiabatic electronic structure data is reviewed. Section 5 describes applications of the ideas presented in Sections 2-4. The photoelectron spectrum of isopropoxy is rationalised. The vibrationally mediated photodissociation dynamics of NH3 in its A state are reinterpreted and the effects of diabolical singularities are described. Section 6 predicts the future. [GRAPHICS] .