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: \[\pi _{\text{sr}}=\frac{\partial q_{\text{s}}}{\partial a_{\text{r}}}\]
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1924 (