You can change the amount of a species similar to a first-order reaction using a first-order rate rule. For example, suppose the species c decays exponentially.
The solution for the rate rule dc/dt = -k*c is .
Enter the rate rule described above and the simulate.
Notice that if the amount of a species c is determined by a rate rule and c is also in a reaction, c must have its BoundaryCondition property set to true. For example, with a reaction a -> c and a rate rule dc/dt = k*c, set the BoundaryCondtion property for c so that a differential rate term is not created from the reaction. The amount of c is determined solely by a differential rate term from the rate rule.
If the BoundaryCondtion property is set to false, you will get the following error message:
Invalid rule variable 'in a reaction or another rule'.