Kohn-Sham density-functional study of low-lying states of the iron clusters Fen+/Fen/Fen- (n=1-4) -: art. no. 155425

All-electron Kohn-Sham density-functional theory calculations have been performed on neutral, cationic, and anionic iron clusters up to the tetramer using local and gradient corrected functionals. Many starting structures were used and various occupation schemes of the molecular orbitals were studied with and without symmetry constraints. A vibrational analysis has been performed on every optimized structure. It is demonstrated that local functionals, in particular, the approximation of Vosko, Wilk, and Nusair, underestimates the total magnetic moment of the cluster compared to various generalized gradient approximation functionals. It is demonstrated that two-dimensional structures (rhombus, square, and rectangle) have to be considered as possible candidates for the ground state of Fe-4(+), Fe-4, and Fe-4(-). A new lowest state is attributed to Fe-2(+) ((8)Delta(u)(+)), Fe-3 ((11)A(2)), Fe-3(+) ((10)A(2)), Fe-3(-) (B-12(1)), Fe-4 ((15)A(')), and Fe-4(-) ((16)A(')). This study suggests that better functionals or high-level post-self-consistent-field calculations are needed to attribute definitely the ground state of small iron clusters. We propose some guidelines for future studies on small transition-metal clusters.