In this thesis, the photochemistry of water on rutile is studied from first principles.
Surface photochemistry of water is important for a variety of applications and may lead to new routes for hydrogen production.
A cluster embedded in a finite point charge field is used as a model surface.
Systematic investigations ensured that parameters like cluster size, basis set and methods are sufficient for a good description of the water/rutile interaction.

It could be shown that a  Ti₉O₁₈Mg₇¹⁴⁺ cluster, recently used for CO photodesorption, is large enough for water adsorption and dissociation, too.
On this cluster, a molecular and a dissociative adsorption form was found, the former being the most stable one.
Analyzing their geometries, five spatial degrees of freedom were identified as important for potential energy surfaces.
Construction of these surfaces is complicated due to the multi-reference character of the wave function in the course of the dissociation process.
Therefore, within the CASSCF approach a variety of active spaces was investigated. Finally, a two-step solution freezing the inactive orbitals paved the way for the potential energy surface.

The potential energy surface of the electronic ground state is very complicated because of different minima and a strong coupling between all degrees of freedom. The surface was constructed from more than 240000 data points and fitted by means of an artificial neural network. This approach proved to be very reliable giving only small errors and a good interpolation.

For the electronically excited state, one electron was removed from the water molecule. This is justified by the experimental observation of a hole attacking the adsorbate.  This state is dominated by strong repulsive interactions between the adsorbate and the surface, as far as the molecular adsorption form is concerned. However, the dissociative form becomes even more favourite.

The potential energy surfaces were used for quantum dynamical simulations describing the adsorbate as a wave packet. The photodesorption of water in three dimensions was calculated within the jumping wave packet approach. The reaction was found to be a prototype of the MGR mechanism with very fast desorbing molecules.
The second reaction studied in this thesis is the photodissociation of water. Here, a simple system with a fixed \OH group was used to study the motion of hydrogen. The dynamics is controlled by a small energy barrier near the Franck--Condon point. This decreases the dissociation probability and leads to pronounced isotope effects.