## Wikipedia - Equazione di Hammett extended Hammett equation

https://doi.org/10.1351/goldbook.E02279
This term applies in a general way to any multiparametric extension of the @H02732@. It is sometimes used specifically for a form of @D01866@ 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]