Complete active space self-consistent field (CASSCF) calculations with aug-cc-pVTZ basis sets were performed for twelve low-lying electronic states of the methyl peroxy radical CH₃O₂ and its positive and negative ions in C_s symmetry. The accuracy of the energies were improved by including dynamic electron correlation by the multiconfiguration second-order perturbation theory (CASPT2) method. All calculated states are valence states. The calculated adiabatic excitation and affinity energies are in reasonable agreement with the experimental data. Potential energy curves (PECs) for O₂-loss dissociation from the ²A" and ²A' states were calculated at the CASPT2//CASSCF level and the electronic states of the O₂ and CH₃ as the dissociation products were carefully determined by checking the relative energies and geometries of the asymptote products along the PECs. The O₂-loss PEC calculations for CH₃O₂ indicate that O₂-loss dissociation occurs from the ²A" and ²A' states leading to CH₃(²A")＋O₂(³A") and CH₃(²A")＋O₂(¹A"), respectively.

Key words: CH₃O₂, complete active space self-consistent field (CASSCF), multiconfiguration second-order perturbation theory (CASPT2), potential energy curve