---

JaxDEM is a lightweight, fully JAX-compatible Python library that empowers researchers and engineers to easily perform high-performance Discrete Element Method (DEM) simulations in 2D and 3D. Every simulation component is written with pure JAX arrays so that you can:

• JIT-compile the entire solver.
• Run thousands of simulations in parallel with vmap.
• Collect trajectories with jax.lax.scan to avoid interrupting the simulation for I/O operations.
• The provided VTK writer understands when you pass it a batched simulation state or a trajectory, and saves every VTK file concurrently.
• Ship the computation seamlessly to CPU/GPU/TPU.
• Interface easily with ML workloads.
• Keep the codebase short, hackable, and fun.

Whether exploring granular materials, designing new manufacturing processes, working on molecular dynamics, or robotics, JaxDEM provides a robust and easily extendable framework to bring your simulations to life.

Example

A minimal simulation with I/O output might look like this:

import jaxdem as jdem
from jaxdem.utils import grid_state
import jax.numpy as jnp

state = grid_state(
    n_per_axis=(10, 10, 10), spacing=0.5, radius=0.1
)  # Initialize particles arranged in a grid
system = jdem.System.create(
    state.dim,
    domain_type="reflect",
    domain_kw=dict(box_size=20.0 * jnp.ones(state.dim)),
)
steps = 1000
n_every = 10
writer = jdem.VTKWriter(save_every=n_every)

for step in range(steps):
    writer.save(state, system)  # does not block until files are on disk
    state, system = jdem.System.step(state, system)

writer.block_until_ready()

However, there is an even simpler way! You can accumulate the trajectory with jax.lax.scan. As VTKWriter understands batch and trajectory axes, you do not have to interleave I/O with computation in a Python loop.

No need to complicate yourself with scan; we already did it for you:

state, system, (traj_state, traj_sys) = system.trajectory_rollout(
    state,
    system,
    n=steps // n_every,  # number of frames
    stride=n_every,  # steps between frames
)

writer.save(
    traj_state, traj_sys, trajectory=True
)  # does not block until files are on disk

Advantages of the second pattern

Feature	Inside-loop I/O	Rollout + One Save
I/O Barrier	None	None
Python ↔ Device Sync	Every save	Only once
Memory Footprint	Single snapshot	n snapshots in RAM

Why is it fast?

1. trajectory_rollout is implemented with jax.lax.scan, the most efficient way to accumulate data inside a JIT-compiled section; no Python overhead is incurred per step.

2. The generated trajectory is still a pure PyTree of arrays, so writer.save can simultaneously dispatch all frames to the thread-pool.

3. The only extra cost is RAM. For large scenes, you can trade memory for speed by increasing n_every (fewer frames kept in memory) or by writing batches of, say, 100 frames at a time.

For more details, visit the Documentation.