Orbital correlation effects in transition metal microclusters

Within the framework of a self-consistent tight-binding LDA + U approach es tablished in the present work, it is argued that orbital correlation (OC) h as crucial effects to very small transition metal microclusters (with numbe r of atoms N typically less than 10) by leading to orbital polarized ground states. Numerical calculations on Fe, Co and Ni diatomic pairs have reveal ed that inclusion of OC changes magnetic properties fundamentally: the inte r-atomic exchange couplings are at least one order smaller than the bulk va lues and change with respect to electron filling from antiferromagnetic (fo r Fe) to ferromagnetic (for Ni), and the magnetic easy axis is usually comp letely different after the OC effect is considered.