The ground state structure of CuO2, which has been discussed extensively in the literature, has been determined using a multiconfigurational SCF method and second-order perturbation theory. In the gas phase, it has been found to have a side-on C-2v structure. However, when introducing effects of a surrounding matrix, the ground state changes to an end-on C-s structure, with an energy 1.2 kcal/mol lower than the side-on structure. Computed harmonic frequencies are in agreement with experiment. MRSDCI calculations have also been performed and they confirm the results obtained at the second-order perturbation theory level. Previous density functional theory studies have failed to predict a correct ground state structure.