A consistent description of the iron dimer spectrum with a correlated single-determinant wave function

We study the iron dimer by using an accurate ansatz for quantum chemical calculations based on a simple variational wave function, defined by a single geminal expanded in molecular orbitals and combined with a real space correlation factor. By means of this approach we predict that, contrary to previous expectations, the neutral ground state is $^7 \Delta$ while the ground state of the anion is $^8 \Sigma_g^-$, hence explaining in a simple way a long standing controversy in the interpretation of the experiments. Moreover, we characterize consistently the states seen in the photoemission spectroscopy by Leopold \emph{et al.}. It is shown that the non-dynamical correlations included in the geminal expansion are relevant to correctly reproduce the energy ordering of the low-lying spin states.