We propose a theoretical model for photocatalytic processes on titanium dioxide, described by its most stable phase and surface, rutile-TiO2(110). The excitation induced by light promotes electrons from the valence band to the conduction band. In this context, one important requirement is having a correct value of the magnitude of the electronic gap. The use of GGA+U or LDA+U functional with an appropriate U value allows this. The U correction has little consequence on the adsorption strength itself on the TiO2(110) surface. For the ground state, it only yields a slight increase of the interaction strength of some test molecules; the surface basicity is somewhat enhanced. This is interpreted by the shift of TiO2 vacant levels. Photoexcitation is taken into account by imposing two unpaired electrons per cell of the same spin. The size of the cell therefore determines the number of excitations per surface area; the larger the cell, the smaller the electron-hole surface concentration and the smaller the energy for electronic excitation. For the excited state, careful attention must be focused on the localization of the excited electron and of the hole which are crucial for the determination of the lowest electronic states and for the surface reactivity. We found that the excited electron is localized on a pentacoordinated surface titanium atom while the hole is shared by two surface oxygen atoms not too far from The electronic levels associated to the reduced titanium atoms are low in energy; the projected density of states is superposed onto the valence band.