The conventional approach to the theoretical study of chemical
reactions has been to study the reactivity of atoms and molecules in
isolation.  However, practical chemistry takes place in solution, at
surfaces and in general under conditions of strong interaction with
the surrounding environment.  Furthermore, thermodynamic parameters
strongly influence the way and the speed at which a reaction proceeds.
Ab-initio molecular dynamics offers a unique tool for studying and
understanding chemical processes as they normally take place in the
laboratory or in industry.  In fact, it combines chemical accuracy
with the possibility of studying the influence of the environment and
the effect of temperature and pressure.  One limiting factor, however,
is the time scale over which most chemical reactions take place, which
far exceeds the simulation time scale.  We shall present a method that
is able to generate classical trajectories with fixed initial and
final boundary conditions.  Our method is based on the minimization of
a suitably defined action.  In combination with the path sampling
method of Chandler et al., it promises to be a powerful tool for
solving the time scale problem in the study of chemical reactions and
in other systems.