Hund's rule in metal clusters: Prediction of high magnetic moment state of Al₁₂Cu from first-principles calculations

Using ab initio pseudopotential plane wave method and generalized gradient approximation for the exchange and correlation energy, we report the finding of high spin ground state of Al₁₂Cu cluster in accordance with the Hund's rule of maximum spin at half-filling. It has perfect icosahedral symmetry and a magnetic moment of 3μ_B, forming an open shell superatom. Further studies of its interaction with an Al atom have led to an electronically closed shell magic cluster with 1.68 eV highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gap such that the added Al atom is incorporated within the cage around the Cu atom.