DFT Study of the Structural and Electronic Properties of Small Ni_n (n = 2-4) Clusters

摘要

All-electron calculations were performed for Ni_2, Ni_3, and Ni_4 clusters and their respective anions and cations using density functional theory techniques as implemented in the DGauss code. Both local and generalized gradient-corrected functionals were employed. Three different multiplicities were examined for each cluster (M = 3, 5, and 7). The calculated ground states were found to have multiplicities of M = 3 for Ni_2 and Ni_3 and M = 5 for Ni_4 with associated magnetic moments per atom of 1.0 #mu#_B for Ni_2 and Ni_4, while Ni_3 shows an isotropic magnetic distribution. The cluster magnetization is enhanced relative to that of the bulk, which is in qualitative agreement with experimental data for clusters larger than 50 atoms. For these smallest Ni_n clusters, there is a mixture of 3d and 4s bonding. The 3d(4s) contribution to the bonding decreases(increases) going from n=2 to n=4, which is reflected by the larger equilibrium bond lengths of Ni_4, as compared to that of Ni_2. The frontier molecular orbitals, the so-called HOMO and LUMO, were studied; they provide insight into the reactivity of these small particles. We also determined the lowest-energy states of anions, Ni_n~- and cations, Ni_n~+. The calculated ionization potentials and electron affinities are in reasonable agreement with experimental observations as well as with other reported theoretical results.