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 Mn(2) 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 sigma(g) orbital, originating from the atomic virtual 4p(z) orbitals, is essential for a proper description of static correlation in the (1)Sigma(g)(+) state of Mn(2). The calculated spectroscopic parameters of the (1)Sigma(g)(+) state agree well with available experimental data. The calculated equilibrium bond lengths are located between 3.24 and 3.50 A, 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 Mn(2) is highlighted.