Understanding the Existence of a Na₂ Dimer in a High-Spin State

The recent observation of a high-spin Na₂ dimer formed on the surface of liquid helium nanodroplets raises some fundamental questions, as the ground state of Na₂ is known to have zero spin. Is it protected against spontaneous dissociation? What is its binding energy and interatomic distance? Is it stable at a higher temperature? Using calculations based on density functional theory (with and without long-range interaction) and coupled cluster methods, CCSD(T), we show that the bonding in the high-spin Na₂ dimer is governed by van der Waals interaction with binding energy (bond length) varying between -0.030 eV (5.108 Å) and -0.192 eV (4.231 Å), depending on the computational method used. Thus, the experimental method used by Kresin and co-workers can be very useful to study larger metastable high-spin clusters such as Li₄ which was predicted in 1985 to have a tetrahedral structure carrying a magnetic moment of 2 μ_B, while its ground state is planar and nonmagnetic.