atom-atom polarizability

https://dx.doi.org/10.1351/goldbook.AT06992
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