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A chemical reaction network specifies chemical species, reactions between them, and their stoichiometric vectors. It can be modelled deterministically by reaction-rate equations or stochastically by molecule-copy-number jumps.

The molecular copy number of a chemical species is the nonnegative integer number of its molecules in a specified reactor or compartment.

The stoichiometric vector of reaction $r$ records its net change in each chemical species. If the copy-number state is $x$, firing the reaction changes it to $x+{\nu }_r$.