https://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 \(r\): \[\pi _{\text{sr}}=\frac{\partial q_{\text{s}}}{\partial a_{\text{r}}}\]