Controversial electronic structures and energies of Fe₂, Fe₂⁺, and Fe₂^- resolved by RASPT2 calculations

The diatomic molecule Fe₂ was investigated using restricted active space second-order perturbation theory (RASPT2). This molecule is very challenging to study computationally because predictions about the ground state and excited states depend sensitively on the choice of the quantum chemical method. For Fe₂ we show that one needs to go beyond a full-valence active space in order to achieve even qualitative agreement with experiment for the dissociation energy, and we also obtain a smooth ground-state potential curve. In addition we report the first multireference study of Fe₂⁺, and Fe₂^- for which we predict an ⁸Σu- ground state, which was not predicted by previous computational studies. By using an active space large enough to remove the most serious deficiencies of previous theoretical work and by explicitly investigating the interpretations of previous experimental results, this study elucidates previous difficulties and provides - for the first time - a qualitatively correct treatment of Fe₂, Fe₂⁺, and Fe₂^- and Fe₂⁺, and Fe₂^- Moreover, this study represents a record in terms of the number or active electrons and active orbitals in the active space, namely 16 electrons in 28 orbitals. Conventional CASPT2 calculations can be performed with at most 16 electrons in 16 orbitals. We were able to overcome this limit by using the RASPT2 formalism.