Electronic structure calculations at the HF, MP2, and MP4 levels of theory, with the 6-31G** basis set, are reported for stationary points on the OH + HO2 singlet potential energy surface. Two particularly important stationary points are the trioxide (H2O3) global minimum and the reaction transition state for O2(1Δ) + H2O formation. For the latter, the MP4 0 K barrier height is 15.2 kcal/mol. Thus, the formation of O2(1Δ) and H2O is predicted to be unimportant, except at highly elevated temperatures. MP2 vibrational frequencies calculated for H2O3 are in good agreement with experiment. Reaction rate theory calculations are performed to assess the effect collisional stabilization of the vibrationally/rotationally excited intermediate H2O3* has on the apparent loss of the OH and HO2 reactants. In the high-pressure limit each of the H2O3* intermediates is collisionally stabilized. However, at intermediate pressures the importance of collisional stabilization depends on the OH + HO2 → H2O3 reaction exothermicity. The MP4 calculations reported here and a previous configuration interaction (CI) calculation place this exothermicity at -22 to -29 kcal/mol at 0 K. With use of these energies, the collisional stabilization of H2O3* at room temperature is predicted to become important only at pressures in excess of 5000 Torr for the commonly used bath gas He. Thus, at atmospheric pressures of an inefficient bath gas like He, the loss of the OH and HO2 reactants is predicted to occur only on the triplet potential energy surface.