The Electronic Structure of Planar Rhombic Co₂O₂

Low-lying valence states of the planar rhombic (D_2h) structure of Co₂O₂ have been computationally studied at density functional theory (DFT) levels, at the singles and doubles coupled-cluster level augmented with a perturbative treatment of the triples (CCSD(T)), at the complete active space self-consistent field (CASSCF) level, and at the second-order complete active space perturbation theory (CASPT2) level. Calculations at the DFT, CCSD(T), and CASSCF levels suggest that the ground state is a high-spin septet state, whereas the CASPT2 calculations yield a singlet ground state (¹A_g). The wave function of the CCSD(T) ground state (⁷B_2u) is dominated by one Slater determinant and is therefore well described using single-reference methods, whereas the configuration interaction coefficient (CI) of the reference determinant (C₀) of the ¹A_g state is only 0.519 at the CASSCF level. High-spin states are stabilized at the DFT level by increasing the amount of Hartree-Fock (HF) exchange in the functional. Static and dynamic correlation effects must be considered to obtain the correct ground state of Co₂O₂. DFT calculations of the magnetically induced current density (MICD) susceptibility of the ¹A_g state using the TPSSh functional show that the molecule sustains a strong diatropic ring current, which has significant contributions from both σ and π orbitals.