An empirical parameter characterizing the exponential temperature dependence of the rate coefficient, , , where is the gas constant and the thermodynamic temperature. The term is also used for threshold energies in electronic potential surfaces, in which case the term requires careful definition. PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 151
PAC, 1993, 65, 2291 (Nomenclature of kinetic methods of analysis (IUPAC Recommendations 1993)) on page 2294
PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1112