extended Hammett equation

https://doi.org/10.1351/goldbook.E02279
This term applies in a general way to any multiparametric extension of the @[email protected] It is sometimes used specifically for a form of @[email protected] 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.
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1114 [Terms] [Paper]