A multireference perturbation theory study on the Fe₂molecule: in quest of the ground state

The and 7Δ u states of the iron dimer, which according to a long-standing debate are the most probable candidates for the assignment of the ground state, have been investigated within the frame of multireference perturbation theory (PC-NEVPT2 and SC-NEVPT3) up to the third order in the energy. At short internuclear distances the 7Δ u state originates from the interaction of a ground-state 5 D Fe atom with an excited-state 5 F, whereas at large distances it results from the van der Waals interaction of two 5 D atoms. At all levels of calculation the term appears to have lower energy than the short-range 7Δ u state and can thus be assigned as the ground state of the iron dimer. The spectroscopic constants of the state (R e = 2.164 Å, ωe = 302.6 cm−1 at the SC-NEVPT3 level of approximation) are in good accordance with the other high level ab initio calculation published thus far.