The nature of binding in the ground state of the scandium dimer

For the study of the nature of binding in the Sc₂ dimer, the ground state, X⁵Σ _u ^–, was calculated by the valence multireference configuration interaction method with single and double excitations plus Davidson correction, MRCISD (+Q), at the complete basis set (CBS) limit. The employment of the C_2v symmetry group, allowed us to obtain the Sc atoms in different states at the dissociation limit. From the Mulliken population analysis and comparison with atomic energies follows that in the ground state Sc₂ dissociates on one Sc in the ground state and the other in the second excited quartet state, ⁴F _u . The spectroscopic parameters of the ground potential curve, obtained at the valence MRCISD (+Q)/CBS level, are: R _e  = 5.20 bohr, D _e  = 50.37 kcal/mol, and ω _e  = 234.5 cm^-1. The obtained value for the harmonic frequency agrees very well with the experimental one, ω _e  = 239.9 cm^-1. The dissociation energy with reference to the dissociation on two Sc in the ground states was estimated as D _e  = 9.98 kcal/mol. In contrast with many other studied transition-metal dimers, which are attributed to the van der Waals bonded molecules, the Sc₂ dimer is stabilized by the covalent bonding on the hybrid atomic orbitals.