First-principles calculations of the magnetic properties of a Co₂ molecule

We present density functional theory (DFT) and complete active space self-consistent field (CASSCF) calculations of a Co₂ magnetic molecule in order to understand results of electron paramagnetic resonance (EPR) measurements. In the Co₂ molecule, one Co atom is six-coordinated (octahedral Co), and the other is four-coordinated (tetrahedral Co). Our CASSCF calculations yield easy-plane magnetic anisotropy for both Co atoms in agreement with the measurements. The calculated ZFS parameter D for the octahedral Co is about eight times larger than the D parameter for the tetrahedral Co. DFT calculations falsely yield easy-axis anisotropy for the octahedral Co. The effective g-factors calculated by CASSCF and the pseudo-spin Hamiltonian approach are close to experimental results. We also calculated the EPR absorption spectrum using quasi-degenerate perturbation theory (QDPT) on top of CASSCF. The QDPT calculations reproduced the polar angle dependence of the EPR signal. However, the azimuthal angle dependence of the EPR signal remains a puzzle. The results show that extra precautions should be taken when using DFT for magnetic molecules.