Helium dimer
= Helium dimer
{title2=$\mathrm{He_2}$}
A very weakly bound van der Waals molecule, observed in cold <helium> expansions. The absence of an ordinary <covalent bond> in the elementary <molecular orbital> model must not be confused with absence of every possible bound dimer.<The weakest bond: experimental observation of helium dimer by Fei Luo and colleagues>{ref}