The role of kinetic energy in chemical binding - II. Contragradience

In a preceding paper, we examined the GI wavefunctions of small molecules and found that the nonclassical or exchange kinetic energy, T^x, dominates the changes in energy involved in chemical binding. Here we examine more closely the changes in T_x with internuclear separation and find that ΔT^x is large for valence orbitals centered on different atoms because of the large region in which the orbitals are contragradient (i.e., have gradients in obtuse directions). In fact for H₂ this contragradience accounts for 93% of the calculated binding energy. In addition, the behavior of T^x and ΔT^x can usually be predicted from consideration of the permutational symmetry (Young tableau) involved in the wavefunction. The concepts developed here provide an alternative interpretation of the nature of the chemical bond.