Topological analysis of electron densities: Is the presence of an atomic interaction line in an equilibrium geometry a sufficient condition for the existence of a chemical bond?

The structure, energetics, and electron density in the inclusion complex of He in adamantane, C10H16, have been studied by density functional theory calculations at the B3LYP6-311++G(2p,2d) level. Topological analysis of the electron density shows that the He atom is connected to the four tertiary C-t atoms in the cage by atomic interaction lines with (3,-1) critical points. The calculated dissociation energy of the complex He@adamantane(g)=adamantane(g) + He(g) of DeltaE = -645 kJ mol(-1) nevertheless shows that the He-C-t interactions are antibonding.