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 σg orbital, originating from the atomic virtual 4pz orbitals, is essential for a proper description of static correlation in the 1Σg+ state of Mn2. The calculated spectroscopic parameters of the 1Σg + 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.