extended Hammett equation

This term applies in a general way to any multiparametric extension of the Hammett equation. It is sometimes used specifically for a form of dual substituent-parameter equation in which the actual value of the correlated property P under the influence of the substituent X is used, rather than the value relative to that for X = H. An intercept term h corresponding to the value of P for X = H is introduced, e.g. \[P=\alpha \ \sigma _{I}+\beta \ \sigma _{R}+h\] The equation may be applied to systems for which the inclusion of further terms to represent other effects, e.g. steric, is appropriate.
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1114 (https://doi.org/10.1351/pac199466051077)