As a clear demonstration of the bonding information that can be garnered from HCN ® HCNO shifts in equilibrium distances, we single out Δre(H-C) for detailed analysis here. The vertical line in the scatter plot of Figure shows that the exact value is Δre(H-C) = -0.00596 Å. This sizable negative value is rather surprising because the carbon atom in HCN is formally sp hybridized, and greater carbon s-character for a shorter σ bond is not possible in HCNO, at least according to the usual valence-bond paradigms. Therefore, Δre(H-C) exposes some complexities in covalent bonding that constitute an interesting test for DFAs.