The following species which does not exist, according to molecular orbital theory.
Helium and the other inert gases have completely filled octets, and so they have zero valency.
That means, their combining capacity is zero, and so they exist as monatomic molecules such as He, Ne, Ar etc.
Helium atom has the electronic configuration 1s2.
According to the molecular orbital theory, in a supposed He2 molecule, both the bonding and the antibonding orbitals will have 2 electrons each.
And so, the bond order (half the difference in the numbers of bonding and antibonding electrons) for such a diatomic entity is zero. That means, the He2 molecule has no existence.