HOSO2 radical is the key intermediate for the oxidation SO2 to SO3 by OH radical in the atmosphere. The structural aspects and the energetics of the reaction HOSO2+O-2--> SO3+HO2 have been studied using Moller-Plesset (MP2) and density functional (DFT) techniques with 6-31G** and triple-zeta, quadruple-zeta, and quintuple-zeta quality basis sets including diffuse basis functions. The detailed theoretical analyses have further revealed that this reaction could proceed through the formation of intermediate complexes and an intramolecular proton transfer like transition state. The energetics of these intermediate reactions has been studied in detail. The use of MP2 methods to study such radical mechanisms had some characteristic symmetry-breaking problem with larger basis sets. This unphysical situation with larger basis set MP2 calculations in this hypervalent system has been explained through the interpretation of the relevant energy surface.