Charge-transfer in Symmetry-Adapted Perturbation Theory

In Symmetry-Adapted Perturbation Theory the charge-transfer energy is normally absorbed into the induction energy, but it can be treated separately. There are two contributions: the long-range or polarization term, which ignores effects due to exchange of electrons between the interacting molecules, and an exchange term. The sum of the two is much smaller than the polarization term, by up to an order of magnitude. The net contribution to hydrogen bond energies is a few kJ mol−1 at equilibrium, and is approximately proportional to the exchange-repulsion energy at other geometries and no more than 10% of it in magnitude.