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.