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Abstract: Several high-level ab initio methods were employed in studies of the narrow singlet-triplet separation of the cyclic form of nickel dioxide (NiO2). It is shown that the complete versions of the locally renormalized coupled cluster method with singles, doubles, and noniterative triples (LR-CCSD(T)) approach, in contrast to the standard CCSD(T) method, provides results in concert with predictions of the density functional theory (DFT) and internally contracted multi-reference configuration interaction method (IC-MRCI), which favor the triplet state to be the lowest one. Relevant discussion of several aspects related to the underlying CCSD calculations, indicate that the dominant role of singly excited amplitudes violates the paradigm about the leading role of two-body effects in the description of the correlation energy. We also show that the multireference perturbation theory, exemplified here by the Generalized Van Vleck Perturbation Theory, requires the use of very large model space in order to properly describe the non-dynamical correlation effects. [Copyright &y& Elsevier]