Conical intersections (CI) are regions of the potential energy landscape
of a chemical system where two or more energy surfaces cross and the
quantum levels are degenerate.
They have an interesting topological structure, and their existence is
signalled by particular geometric properties (the concept of \"Berry phase\"
was introduced in this framework).
Indeed, the importance of CIs has grown in the last years in the field of
photochemistry, since they can represent a very fast reaction channel for the decay of a
photochemically excited system.
An efficient exploration of these gapless regions can help both in
interpreting the experimental data and in suggesting new reaction
mechanisms. To this end, we have developed a strategy based
upon the methods of constrained molecular dynamics and the variational
principles of classical mechanics. This strategy, coupled with high
accuracy quantum chemistry schemes, will be discussed in this seminar,
together with possible applications to realistic systems.