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.

$P = α σ I + β σ R + h$

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