— —   A Density Functional Study of Small Nickel Clusters M. C. MICHELINI, R. PIS DIEZ, A. H. JUBERT ( ) CEQUINOR, Centro de Quımica Inorganica CONICET, UNLP Departamento de Quımica, Facultad ´ ´ ´ de Ciencias Exactas, UNLP, C. C. 962, 1900 La Plata, Argentina Received 22 February 1998; revised 6 May 1998; accepted 25 June 1998 ABSTRACT: Small nickel clusters up to the tetramer are investigated within the framework of the local spin density functional theory. Several competitive states are studied for the dimer. Both the geometry and the spin state are optimized for several starting symmetries in the case of the trimer and the tetramer. Moreover, all those calculations are followed by a vibrational analysis in order to discriminate between real minima and saddle points on the potential energy surface. It is found that Jahn Teller deformations play an important role in determining transition-metal cluster geometries. Equilibrium geometries, electronic configurations, binding energies, magnetic moments, and harmonic frequencies are reported in this work.  1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 693701, 1998 Introduction lusters are useful as models for surfaces, and C as such, they have been used in the analysis of surface processes from a theoretical point of  view 1 . The cluster approach focuses on the prop- erties of a variety of surface sites, taking into account their local geometries and enabling the use of accurate quantum mechanical methods. The rapid development of experimental tech- niques in recent years has made it possible both to obtain size-controlled transition-metal clusters and to study their reactivity against chemisorption pro-   cesses 2, 3 . Although, in principle, metal clusters Correspondence to: A. H. Jubert. and metal surfaces seem to behave in a very simi- lar way against chemisorbed species, small transi- tion-metal clusters show highly size-dependent properties such as their geometries and magnetic  moments 4 . This intriguing behavior can be at- tributed to the fact that transition metals have unfilled valence d orbitals characterized by their localization and high density of states. As a conse- quence of this, a variety of geometries and elec- tronic states are available for a given cluster com- position. Let us say, for example, that the nickel dimer presents almost 30 electronic states located  in an energy range of only 0.75 eV 5 . Because of their important catalytic and mag- netic properties and the just-mentioned complexity of the electronic structure of low-lying excited states, nickel clusters seem to be a challenge for ( ) International Journal of Quantum Chemistry, Vol. 70, 693 701 1998  1998 John Wiley & Sons, Inc. CCC 0020-7608 / 98 / 040693-09