Strongly correlated electrons in the [Ni(hmp)(ROH)X]4 single molecule magnet: a DFT+U study.

The single-molecule magnet [Ni(hmp)(MeOH)Cl]4 (hmp denotes the anion of 2-hydroxymethylpyridine and Me denotes methyl) is studied using both density functional theory (DFT) and the DFT+U method, and the results are compared. By incorporating a Hubbard-U like term for both the nickel and oxygen atoms, the experimentally determined ground state is successfully obtained, and the exchange coupling constants derived from the DFT+U calculation agree with experiment very well. The results show that the nickel 3d and oxygen 2p electrons in this molecule are strongly correlated, and thus the inclusion of on site Coulomb energies is crucial to obtaining the correct results.