free energy: Stochastic simulation algorithm


The stochastic simulation algorithm (SSA) was developed over the twentieth century, like most other MC methods. A sizeable fraction of the literature is dedicated to either improving it's complexity, or implementing it to investigate the properties of Markov processes. Here, the SSA is formulated in the introduction, two versions are written with Python 3 in the implementation section, and these are used in the applications section to simulate a simple epidemic, and enzymatic reaction.


It is likely that the SSA has been well-formulated in probability theory for a good deal of time, but in the context of this discussion, it was first described in 1976 by Daniel T. Gillespie as a systematic, computer-oriented procedure in which rigorously derived Monte Carlo techniques are employed to numerically simulate [Markov processes]. In Gillespie's works, the SSA was implemented to simulate processes known as reactive chemical mixtures. In such processes, a set of chemical species spontaneously form and break bonds with each other following a so-called stoichiometry, which can be quite simple (i.e. the reaction has a single elementary event), or be inoordinately complex (i.e. the reaction has many thousands of elementary events).

In any reactive chemical mixture, each elementary event has a so-called propensity, and this propensity depends on the composition (i.e. the set of chemical species) at any point in time during the reaction. In their works on SSA, Gillespie derives propensity expressions from first principals for certain kinds of elementary events, which are then used to simulate certain reactive chemical mixtures. The extension of propensities to elementary events of any Markov process is generally always possible, but their derivation must be handled with care to produce accurate simulation results (i.e. dimensional analysis must be utilized in a coherent manner).

In the context of reactive chemical mixtures, dependence on the system's composition by propensity $c$ is usually denoted by viewing it as an evaluated function, $c(x_{t})$, where $x_{t}$ is the vector of species counts at the time of the reaction. Further, for one of the countable numbers of elementary events, $j$, it's propensity is denoted $c_{j}(x_{t})$. Now, the so-called fundamental hypothesis of the stochastic formulation of chemical kinetics endows propensities with the following property

$$c_{j}(x_{t}) \delta t + o(\delta t):= \text{average probability, to first order in }\delta t \text{, that elementary reaction }j\text{ will occur in the next time interval }\delta t,$$

where $j$ refers to the index of the elementary event (exercise: give the dimension of $c_{j}(x_{t})$). This probabilistic interpretation of each $c_{j}(x_{t})$ means that $j$ is a random variable, such that

$$p(j) = \frac{c_{j}(x_{t})}{\sum_{j}c_{j}(x_{t})},$$

which can be exploited in a Monte-Carlo step in an alogirthm.

The next consideration in formulating the SSA is to find a probabilistic expression for the time to the next elementary event, $\tau$. In queuing theory $\tau$ is called the sojourn time. In basically every single Markov process, the sojourn time, is exponentially distributed. So then

$$p(\tau) = exp(-\sum_{j}c_{j}(x_{t})\tau),$$

which can also be exploited in a Monte-Carlo step.

The above two probability functions comprise the basic considerations of the SSA, though in the literature they are not introduced in this factorized manner. Notwithstanding, these distribution functions imply the SSA simulates a Markov process by considering two simple, but really interesting random variables-- $j$ (the next elementary event) and $\tau$ (the time, or sojourn, until the next event). Boiling down emperical processes, however complicated, into two simple random variables is one of the most delightful and curious results of numerical science research. Consider next the MC steps for these two random variables.

There is no prescription for how elementary events are ordered as the domain of $p(j)$-- it is in practice usually a matter of convenience. Nevertheless, it is important that each elementary event be hashed to only one $j$ throughout the simulation, and that each elementary event have a fixed place relative to others, so long as it is a valid event. To update populations the following "inversion step" is evaluated with a pseudorandom float, $r$, on $[0,1]$:

$$j = \text{the smallest integer satisfying} \sum_{i=1}^{j} c_{i}(x_{t}) > r\sum_{j}c_{j}(x_{t})$$

To find the sojourn time, the familiar exponential distribution inversion is invoked with another pseudorandom float on $[0,1]$:

$$\tau = \frac{1}{\sum_{j}c_{j}(x_{t})}log_{e}(\frac{1}{r})$$

These MC steps pertain to the so-called "direct-method" of the SSA, which is as follows:

  1. Initialize the time $t = t_{0}$ and systems state to $x=x_{0}$
  2. Evaluate each $c_{j}$ if it depends on $x$ or $t$, and compute $\sum_{j}c_{j}(x_{t})$
  3. Evaluate the Monte Carlo steps to update $t$ and $x$
  4. Update $t$ and $x$ as desired
  5. Return to step 2 or else exit the simulation

There is as well the "first-reaction" method of the SSA, which discounts all the theory behind the direct method presented above (Gillespie tends to motivate the SSA with the direct method). It is as follows (exercise: explain why this version works):

  1. Initialize the time $t = t_{0}$ and systems state to $x=x_{0}$
  2. Draw $j$ pseudorandom numbers and compute a $\hat{\tau}$ with each one
  3. Set $\tau$ as the smallest $\hat{\tau}$, and $j$ as the index of the chosen $\hat{\tau}$
  4. Update $t$ and $x$ as desired
  5. Return to step 2 or else exit the simulation


The SSAs as presented above follow the literautre published by Gillespie, but are naive in at least one respect: they don't specify how to avoid updating the system's categorical populations in nonsensical ways. For example, without certain precautions, it could be possible for a given species' population to dip below zero, which is nonsense. This can be resolved by implementing additional logic that regulates populations, the possible/impossible elementary events, and determines when a simulation should be complete.

A container for all the "unimportant" logic, SSAModel

As a sufficiently general approach to such logic, consider the following "model" class, inhereting from a built-in dictionary, which was designed to offload the control-flow issues described in the above paragraph from the SSAs to itself, thus reducing those tasks to single calls to it's public members. Detailed functionality is given by a full breakdown of each member after the source code.

In [1]:
class SSAModel(dict):
    """Container for SSA model"""

    def __init__(
        """Initialize model"""

        # populate the model

        # set maximum simulation time for any trajectory
        self.max_duration = max_duration

        # cat then sort propens and stoichs in event lists = list()
        self.excluded_events = list()
        for event, propensity in propensities.items():
            if propensity(self) == 0.0:

    def exit(self):
        """Return True to break out of trajectory"""

        # return True if no more reactions
        if len( == 0:
            return True
            return False

        # return True if there is no time left
        if self.max_duration is not None:
            if self["time"][-1] >= self.max_duration:
                return True

    def curate(self):
        """Validate and invalidate elementary events"""
        # evaulate possible reactions
        events = []
        while len( > 0:
            event =
            if event[2](self) == 0:
                events.append(event) = events

        # evaluate impossible reactions
        excluded_events = []
        while len(self.excluded_events) > 0:
            event = self.excluded_events.pop()
            if event[2](self) > 0:
        self.excluded_events = excluded_events

    def reset(self):
        """Clear the trajectory"""

        # reset species to initial conditions
        for key in self:
            del self[key][1:]

        # reset reactions per initial conditions

Member SSAModel.__init__

The constructor accepts four parameters: initial_conditions: dict, propensities: dict, stoichiometry: dict, and max_duration: float or NoneType.

  • The initial_conditions parameter is a dictionary having key-value pairs typed in general like string:numeric[]. An example would be {"species_a": [100], "species_b": [0], "time": [0.0]}. Note that the key "time" must always be included with it's iterable containing an initial time. Any initial condition (for time or otherwise) with more than one specified in it's iterable will have those additional conditions truncated, leading to possibly erroneous results.
  • The propensities dictionary contains $j$ key-value pairs, whereby each key is an integer $\geq 1$ and $\leq j$, and each value is a function that instances of SSAModel can pass themselves to, thus evaluating that elementary event's propensity.
  • The stoichiometry paramater is a dictionary that contains $j$ key-value pairs, in correspondance with the propensities parameter, whereby each value is a dictionary that specifies how to update the model's species.
  • The max_duration parameter defaults to None, but can be a floating point number that specifies when the simulation should end.

After passing in these parameters, SSAModel.__init__ will set the instance's key-value pairs to those specified by the initial_conditions parameter; then it sets the instance's max_duration parameter to the one passed in; then it will initialize the instance's events and excluded_events attributes with corresponding (j, stoichiometry_j, propensity_j) elementary event tuples.

Member SSAModel.exit

This method is called by the SSA with no parameters at the start of each iteration within one simulation (note another word for "one simulation" is "trajectory") to see if the trajectory is complete. Specifically, this method asks it's instance if there are no more possible elementary events or if the trajectory has reached SSAModel.max_duration, and will return True if so. Thus, the iterating loop for a trajectory for the proceding SSA implementation will be of the form while not SSAModel.exit(): ....

Member SSAModel.curate

This method is called by the SSA with no parameters at the end of each iteration within one trajectory to comb out impossible elementary events (setting them in SSAModel.excluded_events), and to reset events back into those that have become possible again.

Member SSAModel.reset

This method is called by the SSA with no parameters at the end of each trajectory to truncate all of SSAModel's values (which again are iterables) to their first, or "inital" value. Exersice: extend SSAModel so that this member accepts a single parameter, n, such that the first $n$ entries of each iterable are not truncated.

Container for the algorithm, SSA

There are many interesting ways to implement the SSA, and there are many interesting implementations that currently exist. The approach herein is to use a class called SSA, that is initialized by passing in a SSAModel instance and a pseudorandom number generator (which we use our dear friend Mersenne), whereby it's members are indefinite generators of SSA trajectories. Here is the implementation (which has generators for the direct-method and first-reaction method), followed by some exercises:

In [2]:
from math import log
from random import random

class SSA:
    """Container for SSAs"""

    def __init__(self, model, seed=1234):
        """Initialize container with model"""
        self.model = model
        self.random = random

    def direct(self):
        """Indefinite generator of direct-method trajectories"""
        while True:
            while not self.model.exit():
                # evaluate weights and partition
                weights = [
                    (rxn, sto, pro(self.model))
                    for (rxn, sto, pro)
                partition = sum(w[-1] for w in weights)

                # evaluate sojourn time
                sojourn = log(
                    1.0 / self.random()
                ) / partition
                    self.model["time"][-1] + sojourn

                # evaluate the event
                partition = partition * self.random()
                while partition >= 0.0:
                    rxn, sto, pro = weights.pop(0)
                    partition -= pro
                for species, delta in sto.items():
                        self.model[species][-1] + delta

            yield self.model
    def first_reaction(self):
        """Indefinite generator of 1st-reaction trajectories"""
        while True:
            while not self.model.exit():

                # evaluate next reaction times
                times = [
                            1.0 / self.random()
                        ) / pro(self.model),
                    for (rxn, sto, pro) in

                # evaluate reaction time
                    self.model["time"][-1] + times[0][0]

                # evaluate reaction
                for species, delta in times[0][1].items():
                        self.model[species][-1] + delta

            yield self.model

Exercise: to each line (except whitespace) in SSA's members, assign the corresponding step number from the direct-method algorithm.

Exercise: to each line ..., assign corresponding step number from the first-reaction-method algorithm.

Exercise: Show how each iterable (except the "time" iterable) of an SSAModel instance is a Markov chain.


A toy epidemic

Regardless of the ontological aspects involved in Gillespie's work deriving the SSA, it is broadly applicable to many observable processes. Consider as an example an epidmeic somewhat similar to the COVID-19 pandemic, in which a population of individuals are all initially in a susceptible state ($S$), and eventually all transition to the infected ($I$) state by acquiring the virus, unless all infected individuals recover before another infection can occur. In the infected state, an individual eithers recover (state $R$) or dies. Further, supposing that the epidemic timescale is significantly shorter than the average lifespan of any individual, it can be assumed that any transitions from the recovered state are not possible. Schematically, here are the possibilities $S \rightarrow I$, $I \rightarrow D$, and $I \rightarrow R$.

These graphs are represented in epidemiology as a system of ODEs, from which propensities can be extracted. Specifically, $S$'s time evolution is governed by $\frac{dS}{dt} = -\alpha S I$, $I$'s time evolution is governed by $\frac{dI}{dt} = \alpha S I - \beta I - \gamma I$, and $R$'s time evolution is governed by $ \frac{dR}{dt} = \gamma I$.

The SSA can be used to simulate if $\alpha$, $\beta$, and $\gamma$ are known empirically (exerice: what are they? State their dimensions). The following simulation is initialized with $500$ individuals, some of which are already infected so as to seed the process (Exercise: if no individual is infected, explain why the simulation will not occur). Included are some helpful comments.

In [3]:
# initial species counts and measurement time
initital_conditions = {
    "s": [480], # susceptible individuals
    "i": [20], # infected individuals
    "r": [0], # recovered individuals
    "d": [0], # dead individuals
    "time": [0.0],

# propensity functions, note
# alpha = 0.01
# beta = 0.1
# gamma = 0.5
propensities = {
    0: lambda d: 0.01 * d["s"][-1] * d["i"][-1],
    1: lambda d: 0.1 * d["i"][-1],
    2: lambda d: 0.5 * d["i"][-1],

# change in species for each propensity
stoichiometry = {
    0: {"s": -1, "i": 1, "r": 0, "d": 0},
    1: {"s": 0, "i": -1, "r": 0, "d": 1},
    2: {"s": 0, "i": -1, "r": 1, "d": 0},

# in event 0, one susceptible individual becomes infected
# in event 1: one infected individual becomes dead
# in event 2: one infected individual becomes recovered

# instantiate the epidemic SSA model
epidemic = SSAModel(

# instantiate the SSA container with model,
# letting the PRNG default to Mersenne
epidemic_generator = SSA(epidemic)

All the basic ingredients necessary to simulate the epidemic are now in place. However, to visualize the results of the simulation, a column of plots is produced (one for each state), and each trajectory is appended to them accordingly--

In [4]:
from matplotlib import pyplot # peculiarity of nbconvert :/
In [5]:
# configure figure object
pyplot.figure(figsize=(10,10), dpi=200)

# make a subplot for susceptible individuals
axes_s = pyplot.subplot(421)

# make a subplot for infected individuals
axes_i = pyplot.subplot(423)

# make a subplot for recovered individuals
axes_r = pyplot.subplot(425)

# make a subplot for deceased individuals
axes_d = pyplot.subplot(427)

# simulate and plot 5 direct-method trajectories
count = 0
for trajectory in
    axes_s.plot(trajectory["time"], trajectory["s"], color="orange")
    axes_i.plot(trajectory["time"], trajectory["i"], color="orange")
    axes_r.plot(trajectory["time"], trajectory["r"], color="orange")
    axes_d.plot(trajectory["time"], trajectory["d"], color="orange")
    count += 1
    if count == 5: break
# make a subplot for susceptible individuals (1st-reaction)
axes_s1 = pyplot.subplot(422)

# make a subplot for infected individuals (1st-reaction)
axes_i1 = pyplot.subplot(424)

# make a subplot for recovered individuals (1st-reaction)
axes_r1 = pyplot.subplot(426)

# make a subplot for deceased individuals (1st-reaction)
axes_d1 = pyplot.subplot(428)

# simulate and plot 5 1st-reaction trajectories
for trajectory in epidemic_generator.first_reaction():
    axes_s1.plot(trajectory["time"], trajectory["s"], color="red")
    axes_i1.plot(trajectory["time"], trajectory["i"], color="red")
    axes_r1.plot(trajectory["time"], trajectory["r"], color="red")
    axes_d1.plot(trajectory["time"], trajectory["d"], color="red")
    count -= 1
    if count == 0: break

In the above, each method is asked to produce 5 trajectories, each a stochastic relaization of the SIR epidemic, with a set of initial conditions (the initial_conditions dictionary), and elementary event propensities and stoichiometries (the propensities and stoichiometry dictionaries).

Enzymatic reaction

The epidemiological example above describes at a macroscopic level how biological materials like a virus interact with populations of biological organisms. On a per-capita basis, a virus works by infecting certain of the organism's cells (i.e. cells of a particular tissue), whereby it enters a cell and integrates it's genetic material such that the cell will synthesize proteins with viral genetic information. These proteins are either viral particles that assemble directly to produce structural facets of the virus, or they are so-called enzymes, which catalyze a chemical reactions related to the production of viral particles in the host cell.

Enzymes also exist in uninfected cells, and they work to support normal cellular activity. Regardless of their purpose, they are in the simplest terms said to operate on one or more substrates to help produce products. In the case of one substrate, an enzyme might interact with it to modify it's bond arrangement or stereochemistry. The enzymes that do this are called isomerases. Theoretically, this process happens as follows. Some number of substrate and some number of corresponding isomerase are floating around in a subcellular compartment, and when one substrate particle interacts appropriately with one isomerase, a substrate-isomerase complex is formed. The isomerase at that point will either catalyze the isomerization and dissociate from the substrate isomer (i.e. the product), or it will dissociate from the substrate without isomerizing it (exercise: with the stoiochiometry dictionary below, draw out the directed graphs for all possible elementary events, supposing $S$ is the substrate, $I$ is the isomerase, $C$ is the substrate-isomerase complex, and $p$ is the product).

The following simulation is initialized with $301$ substrate particles and $120$ isomerase particles (exercise: construct the system of ODEs for this reaction scheme given the propensity and stoichiometry dictionaries below).

In [6]:
# initial species counts and measurement time
initital_conditions = {
    "s": [301], # substrate count
    "i": [120], # isomerase count
    "c": [0], # complex count
    "p": [0], # product count
    "time": [0.0],

# propensity functions for each event
propensities = {
    0: lambda d: 0.0017 * d["s"][-1] * d["i"][-1],
    1: lambda d: 0.0001 * d["c"][-1],
    2: lambda d: 0.1 * d["c"][-1],

# change in species for each event
stoichiometry = {
    0: {"s": -1, "i": -1, "c": 1, "p": 0},
    1: {"s": 1, "i": 1, "c": -1, "p": 0},
    2: {"s": 0, "i": 1, "c": -1, "p": 1},

# instantiate the reaction model
reaction = SSAModel(

# instantiate SSA container
reaction_generator = SSA(reaction)

Similar to the $SIR$ epidemic simulation, plots are initialized and the SSA generators are called below to produce a plot--

In [7]:
# configure figure object
pyplot.figure(figsize=(10,10), dpi=200)

axes_s = pyplot.subplot(421)

axes_i = pyplot.subplot(423)

axes_c = pyplot.subplot(425)

axes_p = pyplot.subplot(427)

# simulate and plot 5 trajectories with direct method
for trajectory in
    axes_s.plot(trajectory["time"], trajectory["s"], color="blue")
    axes_i.plot(trajectory["time"], trajectory["i"], color="blue")
    axes_c.plot(trajectory["time"], trajectory["c"], color="blue")
    axes_p.plot(trajectory["time"], trajectory["p"], color="blue")
    count += 1
    if count == 5: break
axes_s1 = pyplot.subplot(422)

axes_i1 = pyplot.subplot(424)

axes_c1 = pyplot.subplot(426)

axes_p1 = pyplot.subplot(428)

# simulate and plot 5 trajectories with 1st-reaction method
for trajectory in reaction_generator.first_reaction():
    axes_s1.plot(trajectory["time"], trajectory["s"], color="green")
    axes_i1.plot(trajectory["time"], trajectory["i"], color="green")
    axes_c1.plot(trajectory["time"], trajectory["c"], color="green")
    axes_p1.plot(trajectory["time"], trajectory["p"], color="green")
    count -= 1
    if count == 0: break

References and further reading

Gillespie, Daniel T. (2007). "Stochastic Simulation of Chemical Kinetics". The Journal of Physical Chemistry. 58: 35–55.

Gillespie, Daniel T. (1976). "A General Method for Numerically Simulating the Stochastic Time Evolution of Coupled Chemical Reactions". Journal of Computational Physics. 22 (4): 403–434.