Standard ab initio and density functional calculations have been applied to the reaction O-.- + H2O --> [H2O2](.-) --> OH- + OH.. While two intermediates are found as minima on the anionic potential energy surface, neither of them is directly related to the structure of neutral hydrogen peroxide. The results of different combinations of exchange and correlation functionals are systematically compared to each other and to MP2, MP4SDTQ, and CCSD(T) calculated data. The role of the basis set and the Hartree-Fock exchange in the hybrid DFT scheme is discussed. While for the two minima a reasonable agreement between all the methods was found, the geometries of the located transition structures strongly depend on the method and basis set used.