The role of kinetic energy in chemical binding - I. The nonclassical or exchange kinetic energy

The wavefunctions and various partitions of the energy are examined for a variety of small molecules (H₂, H₃, H₄, HeH, HeH₂, He₂, LiH, and BH) in order to isolate the factors crucial for bond formation. We find that a natural partition of the energy leads to the conclusion that the crucial factor is the exchange, or nonclassical, part of the kinetic energy, T^x. The change in T^xupon pushing the atoms towards one another is the dominant term in the binding energy; it is negative when the resulting molecule is stable and positive when it is unstable. We show that T^x is related to the interference kinetic energy considered by Ruedenberg.