The vertical excitation energies for the three lowest excited states of HO2 and the HO2-H2O complex and the molecular orbitals of the HO2 -H2O complex map to the excited states have been calculated with complete active space self-consistent field and multireference configuration interaction (CASSCF) and (MRCI) methods of the corresponding states of HO2. Significant differences between the vertical excited state energies of HO2 and the HO2 -H2O complex are found.