Torch Sim

torch-sim Skill

High-throughput atomistic simulation with ML potentials using torch-sim.

Validated: 2026-01-24 - API tested and working.

When to Use This Skill

Use torch-sim when:

• Screening many structures (10+)
• Need batch GPU acceleration
• Performance is critical
• Using ML potentials (MACE, CHGNet, ORB, etc.)

Use mlip-simulation (ASE-based) when:

• Working with 1-5 structures
• Simplicity matters more than speed
• torch-sim is not available

Rule of thumb: For 10+ structures, prefer torch-sim. For quick single-structure tests, ASE is simpler.

What torch-sim Provides

• 50-100x faster than ASE for batch operations
• PyTorch-native - runs on GPU seamlessly
• Auto-batching - handles memory management
• Multiple MLIPs - MACE, CHGNet, ORB, SevenNet, NequIP, MatterSim

Installation

pip install torch-sim-atomistic

# For specific models:
pip install mace-torch        # MACE
pip install chgnet            # CHGNet
pip install orb-models        # ORB

Verification

python -c "from torch_sim import initialize_state, static; print('torch-sim: OK')"

Model Cache

MACE models cached in: ~/.cache/science-agent/mace/

The demo script automatically downloads models to cache on first run.

Showcase Demo

See: showcases/torch-sim-screening/workspace/demo_torch_sim.py

Core Concepts

Imports

import torch
from torch_sim import initialize_state, static, optimize
from torch_sim import generate_energy_convergence_fn, generate_force_convergence_fn
from torch_sim.models.mace import MaceModel, MaceUrls
from torch_sim.optimizers import Optimizer

Device and Model Setup

IMPORTANT: Models require downloading the file first, then passing the local path:

import urllib.request
from pathlib import Path

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
dtype = torch.float32

# Download model (only needed once)
model_path = Path("./mace_mp_small.model")
if not model_path.exists():
    urllib.request.urlretrieve(MaceUrls.mace_mp_small, model_path)

# Load model
model = MaceModel(str(model_path), device=device, dtype=dtype)

Available MACE URLs:

• MaceUrls.mace_mp_small - Fast, general materials
• MaceUrls.mace_mpa_medium - Better accuracy
• MaceUrls.mace_off_small - Organic molecules

Creating States

IMPORTANT: Use initialize_state(), NOT SimState.from_ase():

from torch_sim import initialize_state
from ase.build import bulk

# Create ASE atoms
structures = [
    bulk('Cu', 'fcc', a=3.6),
    bulk('Al', 'fcc', a=4.05),
    bulk('Ni', 'fcc', a=3.52),
]

# Convert to batched SimState
states = initialize_state(structures, device=device, dtype=dtype)
print(f"Batch size: {states.n_systems}")

High-Level Runners

Static Energy Calculations

IMPORTANT: static() returns list[dict[str, torch.Tensor]], not SimState:

from torch_sim import static

results = static(states, model=model)

# Access results
for i, result in enumerate(results):
    energy = result['potential_energy'].item()  # NOT 'energy'
    forces = result['forces']  # Tensor
    stress = result['stress']  # Tensor
    print(f"Structure {i}: {energy:.4f} eV")

Geometry Optimization

IMPORTANT:

• Use Optimizer.fire enum, not string
• Use convergence functions for criteria
• Returns single state, not tuple

from torch_sim import optimize, generate_energy_convergence_fn
from torch_sim.optimizers import Optimizer

# Create convergence function
convergence_fn = generate_energy_convergence_fn(energy_tol=1e-4)

# Run optimization
final_state = optimize(
    states,
    model=model,
    optimizer=Optimizer.fire,  # or Optimizer.gradient_descent
    convergence_fn=convergence_fn,
    max_steps=500,
)

# Get final energies
final_results = static(final_state, model=model)

Complete Working Example

"""torch-sim batch optimization example"""
import torch
import urllib.request
from pathlib import Path
from ase.build import bulk
from torch_sim import initialize_state, static, optimize, generate_energy_convergence_fn
from torch_sim.models.mace import MaceModel, MaceUrls
from torch_sim.optimizers import Optimizer

# Setup
device = torch.device("cuda")
dtype = torch.float32

# Download and load model
model_path = Path("./mace_mp_small.model")
if not model_path.exists():
    urllib.request.urlretrieve(MaceUrls.mace_mp_small, model_path)
model = MaceModel(str(model_path), device=device, dtype=dtype)

# Create test structures
structures = [
    bulk('Cu', 'fcc', a=3.6),
    bulk('Al', 'fcc', a=4.05),
    bulk('Ni', 'fcc', a=3.52),
    bulk('Fe', 'bcc', a=2.87),
    bulk('W', 'bcc', a=3.16),
]

# Static calculations
states = initialize_state(structures, device=device, dtype=dtype)
results = static(states, model=model)

print("Initial energies:")
for i, result in enumerate(results):
    print(f"  {i}: {result['potential_energy'].item():.4f} eV")

# Optimization
states = initialize_state(structures, device=device, dtype=dtype)
convergence_fn = generate_energy_convergence_fn(energy_tol=1e-4)

final_state = optimize(
    states,
    model=model,
    optimizer=Optimizer.fire,
    convergence_fn=convergence_fn,
    max_steps=500,
)

final_results = static(final_state, model=model)
print("\nOptimized energies:")
for i, result in enumerate(final_results):
    print(f"  {i}: {result['potential_energy'].item():.4f} eV")

Autobatching

For large batches, use autobatchers to manage GPU memory:

from torch_sim.autobatching import InFlightAutoBatcher

autobatcher = InFlightAutoBatcher(memory_fraction=0.8)

final_state = optimize(
    states,
    model=model,
    optimizer=Optimizer.fire,
    autobatcher=autobatcher,
    max_steps=500,
)

Convergence Functions

Two options for convergence criteria:

from torch_sim import generate_energy_convergence_fn, generate_force_convergence_fn

# Energy-based (default) - converge when energy change < tolerance
convergence_fn = generate_energy_convergence_fn(energy_tol=1e-4)

# Force-based - converge when max force < tolerance
# NOTE: Requires cell_forces in state, may not work with all models
convergence_fn = generate_force_convergence_fn(force_tol=0.05)

Converting Results Back

# SimState to ASE Atoms
atoms_list = final_state.to_atoms()

# SimState to Pymatgen Structures
structures = final_state.to_structures()

Performance (Validated on RTX 5080)

Comparison with ASE

Common Issues

Model Loading Error

FileNotFoundError: ...

Solution: Download model file first, don't pass URL directly to MaceModel.

Missing cell_forces

ValueError: cell_forces not found in state

Solution: Use generate_energy_convergence_fn() instead of generate_force_convergence_fn().

Unpack Error

ValueError: too many values to unpack

Solution: optimize() returns single state, not tuple.

References

• torch-sim Documentation
• torch-sim GitHub
• MACE Models

See Also

• mlip-simulation skill - Simpler ASE-based ML potential usage
• data-analysis skill - Analyzing torch-sim outputs