On the Existence of HeHe Bond in the Endohedral Fullerene Hе₂ @C₆₀

Twenty years have already been passed since the endohedral fullerene's void ceaselessly attracts attention of both, experimentalists and theoreticians, computational chemists and physicists in particular, who direct their efforts on computer simulations of encapsulating atoms and molecules into fullerene void and on unraveling the arising bonding patterns. We review recent developments on the endohedral He₂ @C₆₀ fullerene, on its experimental observation and on related computational works. The two latter are the main concerns in the present work: on the one hand, there experimentally exists the He dimer embedded into C₆₀ void. On the other, computational side, each He atom exhibits a negligible charge transfer to C₆₀ resulting in that altogether, the He dimer exists as a fractionally charged (He^+δ )₂ . Whether there exists a bond between these two helium atoms is the key question of the present work. Since a bond is a two-body creature, we assert that it suffices to define the bond on the basis of Löwdin's postulate of a molecule which we invoke to investigate such formation of the He dimer in a given C₆₀ void in terms of the HeHe potential energy well. It is analytically demonstrated that this well enables to maintain at least one bound (ground) state, and therefore, according to Löwdin's postulate which is naturally anticipated within quantum theory, we infer that (He^+δ )₂ is a molecule, a diatomic, where two heliums are bonded to each other. Using these arguments, we also propose to extend the concept of stability of endohedral fullerenes. © 2017 Wiley Periodicals, Inc.