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 Mn-2, 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 R-e of 3.7-3.8 A, by a low dissociation energy D-e of 0.07-0.08 eV, and by a small harmonic frequency omega(e) of 43 cm(-1). The experimental evidence that Mn-2 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 R-e=2.50 A, D-e=1.35 eV, and omega(e)=246 cm(-1). (c) 2008 American Institute of Physics.