Magnetism of a four-center transition-metal cluster revisited

Abstract

We propose a surprisingly efficient procedure to map a sophisticated treatment of electronic correlations to a few-parameter model Hamiltonian. By pushing first-principles coupled-cluster methods to the limit we get a lucid picture of the electronic excitations responsible for the magnetic interactions in a paradigm ${\mathrm{Ni}}_{4}$ cluster. Using an extended Hubbard model, we demonstrate that the magnetism in this system cannot be explained solely on the basis of spin models but additionally requires consideration of the indirect mechanism involving hopping and on-site repulsion. We present a protocol to extract the corresponding parameters from experiment or calculations.