# FAQs

## How to disable rescaling of reaction rates in rate laws?

As explained in the Reaction rate laws used in simulations section, for a reaction such as k, 2X --> 0, the generated rate law will rescale the rate constant, giving k*X^2/2 instead of k*X^2 for ODEs and k*X*(X-1)/2 instead of k*X*(X-1) for jumps. This can be disabled when directly converting a ReactionSystem. If rn is a generated ReactionSystem, we can do

osys = convert(ODESystem, rn; combinatoric_ratelaws=false)

Disabling these rescalings should work for all conversions of ReactionSystems to other ModelingToolkit.AbstractSystems.

## How to modify generated ODEs?

Conversion to other ModelingToolkit.AbstractSystems allows the possibility to modify the system with further terms that are difficult to encode as a chemical reaction. For example, suppose we wish to add a forcing term, $10\sin(10t)$, to the ODE for dX/dt above. We can do so as:

dXdteq = equations(osys)
t      = get_iv(osys)
dXdteq = Equation(dXdteq.lhs, dXdteq.rhs + 10*sin(10*t))
@named osys2  = ODESystem([dXdteq], t, states(osys), parameters(osys))
oprob  = ODEProblem(osys2, u0map, tspan, pmap)
osol   = solve(oprob, Tsit5())

We can add $e^{-X}$ to $dX/dt$ as a forcing term by

dXdteq = equations(osys)
@variables X(t)
dXdteq = Equation(dXdteq.lhs, dXdteq.rhs + exp(-X))
@named osys2  = ODESystem([dXdteq], t, states(osys), parameters(osys))
oprob  = ODEProblem(osys2, u0map, tspan, pmap)
osol   = solve(oprob, Tsit5())

## How to override mass action kinetics rate laws?

While generally one wants the reaction rate law to use the law of mass action, so the reaction

rn = @reaction_network begin
k, X --> ∅
end k

occurs at the (ODE) rate $d[X]/dt = -k[X]$, it is possible to override this by using any of the following non-filled arrows when declaring the reaction: ⇐, ⟽, ⇒, ⟾, ⇔, ⟺. This means that the reaction

rn = @reaction_network begin
k, X ⇒ ∅
end k

will occur at rate $d[X]/dt = -k$ (which might become a problem since $[X]$ will be degraded at a constant rate even when very small or equal to 0).

Note, stoichiometric coefficients are still included, i.e. the reaction

rn = @reaction_network begin
k, 2*X ⇒ ∅
end k

has rate $d[X]/dt = -2 k$.

## How to specify user defined functions as reaction rates?

The reaction network DSL can "see" user defined functions that work with ModelingToolkit. e.g., this is should work

myHill(x) = 2.0*x^3/(x^3+1.5^3)
rn = @reaction_network begin
myHill(X), ∅ → X
end

In some cases, it may be necessary or desirable to register functions with Symbolics.jl before their use in Catalyst, see the discussion here.