Résumé
Chemistry that takes place exclusively in the ground electronic state can be well described by a reaction path in which the reactants pass over a transition state to the products. After photoexcitation, a molecule is in an excited electronic state and new topographical features joining different states, known as conical intersections, also need to be considered to describe the time-evolution from reactants to products. These intersections are due to the coupling between electrons and nuclei. In addition to providing new pathways, they provide a quantum-mechanical phase to the system which means that to describe the nuclear motion properly methods are required that include the resulting quantum-mechanical coherences in the nuclear motion. In this chapter, we review the nature and topography of conical intersections and simulation methods that have been developed to describe a molecule passing through one. These range from the full solution of the time-dependent Schrodinger equation to approximate methods based on Newtonian mechanics. Using examples the advantages and disadvantages of each are discussed.