crn-jax 0.1

Chemical reaction networks in JAX: GPU-parallel stochastic simulations.

crn-jax

Chemical reaction networks in JAX — a tiny, GPU-optimized Gillespie / Stochastic Simulation Algorithm (SSA) library.

 

Wall-time to simulate 1,000,000 independent stochastic trajectories — each a full Gillespie run of the reaction network from t=0 to t=20, sampled at 200 time points (CPU vs RTX 5090 GPU).

Install

pip install crn-jax

# with NVIDIA GPU support:
pip install crn-jax "jax[cuda12]"

# with plotting helpers:
pip install "crn-jax[examples]"

# for local development (uses Poetry):
git clone https://github.com/robinhenry/crn-jax && cd crn-jax
poetry install                    # main deps + dev tools
poetry install --with gpu         # add jax[cuda12] on an NVIDIA host

crn-jax depends on jax / jaxlib only.

Key features

• 🎯 Exact SSA — pure-JAX implementation of the Gillespie algorithm for chemical reaction networks.
• ⚡ JIT-compiled — the entire loop compiles under jax.jit.
• 🚀 GPU speedup — 1M+ independent trajectories on a single GPU under jax.vmap, with no Python overhead.
• ⏱️ Discretization-safe — pending reaction times are preserved across simulation-interval boundaries, so trajectories are physically correct under discrete observations (or fixed-interval stepping).
• 🎛️ Control-input aware — propensities take an optional input argument that can vary per-interval and per-replicate, so each of N parallel trajectories can follow its own control schedule (useful for RL-style rollouts, closed-loop experiments with per-replicate inputs, …).
• 🧩 Bring-your-own state — the loop operates on any PyTree (NamedTuple, Flax struct dataclass, Equinox module, …).

Quickstart

A 1-species birth-death process, ∅ → X at rate λ and X → ∅ at rate μ·x, simulated for 10 independent replicates and plotted:

from typing import NamedTuple
import jax, jax.numpy as jnp
from crn_jax import simulate_trajectory, plot_trajectories

BIRTH_RATE, DEATH_RATE = 3.0, 0.1    # steady-state mean λ/μ = 30

# Define a state-holding object
class State(NamedTuple):
    time: jax.Array
    x: jax.Array
    next_reaction_time: jax.Array    # carried across intervals

# Return propensity equations as an array
# with an optional external input (unused here)
def propensities(s, _input):
    return jnp.array([BIRTH_RATE, DEATH_RATE * s.x])

# Describe how the state changes when reaction `j` fires
def apply_reaction(s, j):
    return s._replace(x=s.x + jnp.where(j == 0, 1.0, -1.0))

# Initial state
state0 = State(jnp.array(0.0), jnp.array(0.0), jnp.array(jnp.inf))

@jax.jit
@jax.vmap
def run_one(key):
    return simulate_trajectory(
        key=key,
        initial_state=state0,
        timestep=1.0,
        n_steps=200,
        # Pass our 2 custom functions defined above
        compute_propensities_fn=propensities,
        apply_reaction_fn=apply_reaction,
    )

# Simulate 100 Gillespie trajectories
states = run_one(jax.random.split(jax.random.PRNGKey(0), 100))
times = jnp.arange(1, 201) * 1.0

See the examples folder for more detailed examples.

API

# Main entry point: scan n_steps fixed-length intervals, stack the per-step states.
from crn_jax import simulate_trajectory

# Finer control: one interval at a time (RL-style), or until an absolute time.
from crn_jax.gillespie import simulate_interval, simulate_until

# Plotting helper: step-plots a single trajectory or an (N, T) ensemble.
from crn_jax import plot_trajectories

# Optional kinetic-law helpers.
from crn_jax.kinetics import hill_function, sample_lognormal

function	when to reach for it
simulate_trajectory	You want a full trajectory on a fixed sampling grid. Start here.
simulate_interval	You're driving the system yourself, one step at a time (e.g. an RL rollout).
simulate_until	You need a custom state shape or a non-uniform time grid. Fully generic.
plot_trajectories	Quick look at the output.

See Also

• GillesPy2: C++ optimized Gillespie simulations on CPU.
• myriad-jax: RL-style decision making fully in JAX, powered by grn-jax at its core.