atom-atom polarizability
A quantity used in perturbation HMO theory as a measure of the change in electron density, q, of atom s caused by a change in the electronegativity (or Coulomb integral) of atom r:
π sr = ∂ q s ∂ a r
PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1924