Projected unrestricted Hartree-Fock calculations and the magnetism of large nickel clusters

The study of the multiplicity (M) of transition metal clusters of different sizes has been motivated by its importance in relation to the reactivity o f the structures. For this reason, the accuracy and utility of restricted a nd unrestricted HF methodologies have been extensively analyzed. We compare rather time-consuming restricted open-shell Hartree-Fock calculations foll owed by multireference CI (ROHF/MRCI) with fully projected unrestricted Har tree-Fock (PUHF) methodologies for the evaluation of the M of small Ni, clu sters (n = 4, 6, 8, 13) using the intermediate neglect of differential over lap model Hamiltonian (INDO). Different geometry and lattice parameters are considered, searching for the best way of dealing with the open-shell elec tronic distributions that are strongly related to reactivity. We examine bo th the optimized, most stable structures, and those associated with the obs erved interatomic bulk distance. Our results are compared with those obtain ed from density functional and ab initio calculations, as well as with expe rimental data, when available. On the basis of the results of this comparis on, the PUHF model is applied to the study of trilayer and bilayer surface- slab Ni clusters of 20 and 51 atoms, respectively.