A multiconfiguration time-dependent Hartree method is applied to study the photodissociation dynamics of methyl iodide on a MgO surface. The surface is; assumed : to be rigid and the dissociation is restricted in orientations parallel to the surface normal. Two active mathematical dimensions and two electronic states are included in our model. The dynamics of the fragments (the methyl radical and iodine atom) are;complicated by the so-called "chattering effect" and by nonadiabatic transitions between the two dissociative states of CH3I. It is shown from the comparison with exact calculations that the single configuration time-dependent Hartree approximation fails to give an accurate description of the dynamics. The failure of the Hartree approximation, which is due to the neglect of direct spatial correlations between modes, can be effectively corrected by including a few additional configurations. Our results show that four configurations are sufficient to provide an accurate description of the complex dissociation dynamics of the CH3I/MgO system.