  This Chapter is devoted to unravel the relaxation processes taking place
after photoexcitation of isolated DNA/RNA nucleobases in gas phase from a
time-dependent perspective. To this aim, several methods are at hand, ranging
from full quantum dynamics to various flavours of semiclassical or ab initio
molecular dynamics, each with its advantages and its limitations. As this
contribution shows, the most common approach employed up-to-date to learn about
the deactivation of nucleobases in gas phase is a combination of the Tully
surface hopping algorithm with on-the-fly CASSCF calculations. Different
methods or, even more dramatically, different electronic structure methods can
provide different dynamics. A comprehensive review of the different mechanisms
suggested for each nucleobase is provided and compared to available
experimental time scales. The results are discussed in a general context
involving the effects of the different applied electronic structure and
dynamics methods. Mechanistic similarities and differences between the two
groups of nucleobases---the purine derivatives (adenine and guanine) and the
pyrimidine derivatives (thymine, uracil, and cytosine)---are elucidated.
Finally, a perspective on the future of dynamics simulations in the context of
nucleobase relaxation is given.
