# Documentation

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## Create a Rate Rule for an Exponential Rate of Change

This example shows how to change the amount of a species similar to a first-order reaction using the first-order rate rule. For example, suppose the species `x` decays exponentially. The rate of change of species `x` is:

The analytical solution is:

where is the amount of species at time t, and is the initial amount. Use the following commands to set up a SimBiology model accordingly and simulate it.

```m = sbiomodel('m'); c = addcompartment(m,'comp'); s = addspecies(m,'x','InitialAmount',2); p = addparameter(m,'k','Value',1); r = addrule(m,'x = -k * x','RuleType','rate'); [t,sd,species] = sbiosimulate(m); plot(t,sd); legend(species); xlabel('Time'); ylabel('Species Amount');```

If the amount of a species `x` is determined by a rate rule and `x` is also in a reaction, `x` must have its `BoundaryCondition` property set to `true`. For example, with a reaction `a -> x` and a rate rule , set the `BoundaryCondition` property of species `x` to `true` so that a differential rate term is not created from the reaction. The amount of `x` is determined solely by a differential rate term from the rate rule. If the `BoundaryCondition` property is set to `false`, you will get the following error message such as `Invalid rule variable 'x' in rate rule or reaction`.

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