A multireference perturbation theory study on the Fe2 molecule: in quest of the ground state

The (9)Sigma(g) and (7)Delta(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 (7)Delta(u) state originates from the interaction of a ground-state D-5 Fe atom with an excited-state F-5, whereas at large distances it results from the van der Waals interaction of two D-5 atoms. At all levels of calculation the (9)Sigma(-)(g) term appears to have lower energy than the short-range (7)Delta(u) state and can thus be assigned as the ground state of the iron dimer. The spectroscopic constants of the (9)Sigma(-)(g) state (R-e = 2.164 angstrom, omega(e) = 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.