A theoretical study of the electronic structure of the Co2O2 molecule

The electronic structure of the Co2O2 molecule in its planar rhombic form has been studied theoretically by means of DFT and wave function based ab initio calculations. In the DFT part, based on the broken symmetry approach and the TPSS functional, four low-lying spin states were investigated, one singlet, triplet, quintet, and septet. The singlet state was found to have the lowest energy, its calculated equilibrium geometry and vibration spectrum are in fair agreement with the respective experimental data. Wave function based ab initio calculations at increasing level of accuracy (CASSCF, CAS-CI, MC-CEPA, and SOC-CI) were used to determine the whole manifold of low-lying electronic states. Without inclusion of spin-orbit coupling, several low-lying spin ladders exist, each consisting of a singlet, a triplet, a quintet, and a septet state. The spin coupling is antiferromagnetic in the four lowest ladders, with similar exchange coupling constants J of about −60 cm−1 at the CASSCF and −120 cm−1 at the correlated level. The overall ground state has the symmetry 1Ag. Spin-orbit coupling has a strong effect on the order and energetics of the lowest levels: after its inclusion a nearly degenerate pair of states is found to be lowest in energy, followed by a second pair 300 cm−1 higher in energy and another bunch of six states after another 200 cm−1.