An ab initio multireference perturbation theory study on the manganese dimer.

The potential energy curves of the ground state and of some excited states of the manganese dimer have been calculated over a wide range of internuclear distances using the second order n-electron valence state perturbation theory applied to a complete active space self-consistent field reference wave function. The ground state of Mn2, for which also the third order NEVPT has been used, is calculated to be a singlet belonging to the Sigma_g^+ symmetry, characterized by a large equilibrium internuclear distance Re of 3.7–3.8 Å, by a low dissociation energy De of 0.07–0.08 eV, and by a small harmonic frequency omega_e of 43 cm−1. The experimental evidence that Mn2 is a van der Waals molecule is thus confirmed. Among the excited states, {^11}Pi_u, which is usually indicated as the ground state by density functional theory studies, appears as a low-lying state with Re=2.50 Å, De=1.35 eV, and omega_e=246 cm−1.