bond/react: Arrhenius constraint

doc/src/Eqs/fix_bond_react.tex

@@ -0,0 +1,9 @@
+\documentstyle[12pt]{article}
+\pagestyle{empty}
+\begin{document}
+
+\begin{eqnarray*}
+   k = AT^{n}e^{\frac{-E_{a}}{k_{B}T}}
+\end{eqnarray*}                           
+
+\end{document}

doc/src/fix_bond_react.txt

@@ -266,7 +266,7 @@ either 'none' or 'charges.' Further details are provided in the
 discussion of the 'update_edges' keyword. The fourth optional section
 begins with the keyword 'Constraints' and lists additional criteria
 that must be satisfied in order for the reaction to occur. Currently,
-there are two types of constraints available, as discussed below.
+there are three types of constraints available, as discussed below.
 
 A sample map file is given below:
 
@@ -320,6 +320,27 @@ the central atom). Angles must be specified in degrees. This
 constraint can be used to enforce a certain orientation between
 reacting molecules.
 
+The constraint of type 'arrhenius' imposes an additional reaction
+probability according to the temperature-dependent Arrhenius equation:
+
+:c,image(Eqs/fix_bond_react.jpg)
+
+The Arrhenius constraint has the following syntax:
+
+arrhenius {A} {n} {E_a} {seed} :pre
+
+where 'arrhenius' is the required keyword, {A} is the pre-exponential
+factor, {n} is the exponent of the temperature dependence, {E_a} is
+the activation energy ("units"_units.html of energy), and {seed} is a
+random number seed. The temperature is defined as the instantaneous
+temperature averaged over all atoms in the reaction site, and is
+calculated in the same manner as for example
+"compute_temp_chunk"_compute_temp_chunk.html. Currently, there are no
+options for additional temperature averaging or velocity-biased
+temperature calculations. A uniform random number between 0 and 1 is
+generated using {seed}; if this number is less than the result of the
+Arrhenius equation above, the reaction is permitted occur.
+
 Once a reaction site has been successfully identified, data structures
 within LAMMPS that store bond topology are updated to reflect the
 post-reacted molecule template. All force fields with fixed bonds,