The derivation of the SSA is relatively straightforward. For a well-stirred chemical system with 𝑁 reactions, starting from the reaction probability density function and applying limits as well as solving ordinary differential equations (ODEs), it can be shown that the time interval between two successive reactions follows an exponential distribution with a parameter ∑ 𝑁 𝑎 𝑖 . Here, 𝑎 𝑖 , referred to as the propensity function, represents the microscopic reaction rate for the 𝑖-th reaction.