A QSAR equation that describes the relationship between the logarithm of the relative biological potency of a molecule (\(\log (1/C_{i})\)) and its electronic and hydrophobic properties: \[\log (1/C_{i}) = \rho \sigma_{i} + b\,\log P_{i} - c\,(\log P_{i})^{2}\] in which fitting to the square term provides a parabola with an associated optimum log P.