Quantum Chemical Study of Nickelocene: From Magnetic Molecules to Materials

A reliable ab initio description of molecular magnets is key to developing novel molecule-based quantum devices, with the potential to be more efficient and easily tunable. However, quantum chemical calculations of such magnetic materials are limited to density functional theory (DFT) or DFT+U (Hubbard correction to DFT). We employ our recently developed protocol based on the equation-of-motion coupled-cluster (EOM-CC) framework to investigate magnetic behavior of nickelocene (NiCp₂, Cp = cyclopentadienyl) molecular magnet. The protocol is implemented within the ezMagnet software. Our calculations agree well with experimentally derived magnetic anisotropy and susceptibility values. The calculations show that magnetic anisotropy in NiCp₂ originates from the spin-orbit coupling between the ground state and the third singlet state, instead of the closest-lying singlet state. Benchmarking DFT against EOM-CC enables then reliable DFT calculations on a realistic model of the NiCp₂/MgO(001) absorption complex. The analysis of the resulting spinless transition density matrices and their natural transition orbitals explains how magnetic properties of NiCp₂ are retained upon adsorption on MgO, making NiCp₂ attractive as a spin sensor. The protocol is general and can be combined with density embedding techniques, allowing us to investigate systems as large and complex as molecular magnets on metal surfaces and their self-assemblies.