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 $r$ :

$π sr = ∂ q s ∂ a r$

Source:

PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1924