The Electronic Structure of Planar Rhombic Co2O2

Low-lying valence states of the planar rhombic (D2h) 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 (1Ag). The wave function of the CCSD(T) ground state (7B2u) 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 1Ag 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 1Ag 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.