Choosing a proper complete active space in calculations for transition metal dimers: Ground state of Mn₂ revisited

The potential energy curve of the ground state of Mn₂ has been studied using a systematic sequence of complete active spaces. Deficiencies of the routinely used active space, built from atomic 4s and 3d orbitals, has been identified and discussed. It is shown that an additional σ_g orbital, originating from the atomic virtual 4p_z orbitals, is essential for a proper description of static correlation in the ¹Σ_g⁺ state of Mn₂. The calculated spectroscopic parameters of the ¹Σ_g ⁺ state agree well with available experimental data. The calculated equilibrium bond lengths are located between 3.24 and 3.50 Å, the harmonic vibrational frequencies, between 44 and 72 cm^-1, and the dissociation energies, between 0.05 and 0.09 eV. An urgent need for an accurate gas-phase experimental study of spectroscopic constants of Mn₂ is highlighted.