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Abstract: The 12Π(X2Π), 22Π, 12Σ+, 32Π, 42Π, and 22Σ+ states of the OCS+ ion were studied using the CASSCF and CASPT2 methods with a contracted ANO basis and 32Π and 42Π are characterized as shake-up ionization states. The CASSCF frequency calculations indicate that the 12Π, 22Π, 12Σ+, and 22Σ+ primary ionization states represent minima in the potential energy surfaces (PESs), and accurate CASPT2 adiabatic excitation energy (T 0) calculations support assignments of the A, B, and C states to 22Π, 12Σ+, and 22Σ+, respectively. The CASSCF frequency calculations indicate that 32Π and 42Π represent saddle points in the PESs. We found minima in the PESs of the 52A′ and 42A″ states (related to 42Π), and the CASPT2 T 0 values for 52A′ and 42A″ are between the T 0 values of B2Σ+ and C 2Σ+. The observed satellite bands between the B and C bands in spectrum can be (partially) attributed to 52A′ and 42A″. [Copyright &y& Elsevier]