On 30/11/11 08:50, Chris J. Myers wrote:
> Hi All,
> Currently, the SBML specification does not describe how to perform stochastic simulation on an SBML model. This has led to different tools implementing it differently making stochastic models not portable from one tool to another. Therefore, we would like to come up with a precise semantics for stochastic simulation to solve this problem. There is a subtlety though that complicates things. Consider the following model:
> Species A has an initial amount of 1 mole while species B has an initial amount of 0 moles. There is one reaction with A as a reactant and B as a product. The question is what happens to the state when this reaction occurs.
> 1) 1 mole of A is converted into 1 mole of B
> Pro: simple interpretation that is independent of units (namely, units can be dropped without any change in behavior).
> Con: typical interpretation of stochastic formulations is that reactions update molecule counts.
> 2) 1 molecule of A is converted into 1 molecule of B
> Pro: matches the typical interpretation.
> Con: the behavior is no longer independent of units (namely, a unit conversion must be performed implicitly).
> In an implementation of (2), one would need to convert the species initial amounts into units of individual molecule counts. In the case of moles, this would mean multiplying each species by Avogadro's number (6.02e23). If the units are changed to nanoMoles, then a different conversion is necessary.
I think that is the behaviour to be expected; the meaning of the model
(once variables are converted into common units) should not change just
because the units have changed.
For example, a model with numbers in nM should produce the same results
as a model with numbers in M, once the numbers have been converted into
nM. I therefore think that the units behaviour is a con for 1 and a pro
for 2, and therefore option 1 has both cons, and option 2 should be picked.
> So, the question is which is more important:
> 1) Avoiding having implicit unit conversions and having units affect simulation behavior.
> 2) Keeping an individual molecule interpretation of stochastic reactions.
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