In this work we have carried out ab initio complete active space self-consistent-field (CASSCF) calculations, second-order perturbation calculations based on CASSCF wave functions (CASPT2), uncontracted multireference configuration interaction calculations, and some density functional calculations with standard correlation-consistent Dunning basis sets and atomic natural orbital basis sets on the lowest (2)A' and (4)A' potential energy surfaces involved in the title reaction. The ground (2)A' surface has an average energy barrier of 5.3 kcal/mol in the CASPT2 complete basis set limit. A peroxy NOO minimum is found in agreement with preceding ab initio works, which seems to play an important role in the opening of a double microscopic mechanism: direct C-s abstraction and indirect C-s insertion through the NO2(X (2)A(1)) molecule. The ground (4)A' surface shows an average energy barrier of 13.5 kcal/mol in the CASPT2 complete basis set limit. Despite this excited surface displays another peroxy minimum, in this case only a direct C-s-abstraction mechanism can be expected. The present results improve previous high quality ab initio studies and provide lower energy barriers in both potential energy surfaces, which would produce larger total thermal rate constants in better agreement with experimental data. Finally, it is demonstrated that the N and O 2s electron correlation cannot be neglected as it produces a significant decrease in both energy barriers.