Molecular Dynamics

Goal

To perform stable and accurate molecular dynamics simulations using MLIPs, ensuring physical correctness and avoiding common "explosions" associated with neural network potentials.

Instructions

1. Monitoring Stability

MD stability monitoring is integrated directly into the run_md tool via ASE callbacks. This ensures zero-latency response to instabilities and simplifies the simulation workflow.

• Enable Monitoring: Set monitor=True and specify monitor_type (single string or list).

  • explosion: Safety check. Stops if T > 10,000K or NaN. Recommended for all unstable simulations.
  • equilibration: Convergence check. Stops once temperature and potential energy stabilize (e.g., for production runs).
  • overshoot: Thermostat check. Stops if T deviates significantly from target (T-target > 200K).
  • volume: NPT stability check. Stops if volume expands by 2x or contracts to 0.2x of initial.
  • diffusion: Convergence check for transport properties. Stops once the relative error of diffusivity for a specific specie (default Li) falls below a threshold (default 0.1).
    • Parameters: specie, threshold, check_interval_ps (default 5.0), ignore_ps (initial equilibration to skip, default 5.0).
  • quenching: Linear temperature ramp. Updates the thermostat target every step to move from temperature to temperature_end over a specified number of steps.
    • Best Practice: Use dyn.set_temperature(temperature_K=T) inside the ramping callback. This is critical for thermostats like Langevin to update internal noise/coupling coefficients.
    • Advanced Thermostats: For NoseHooverChainNVT and MTKNPT, where set_temperature might be missing, manual updates to internal attributes (_kT, _Q, _W) are required to keep the damping frequency consistent.

• Example Usage:

  # MACE example with multiple monitors
  mace.run_md(structure, monitor=True, monitor_type=["explosion", "equilibration"])
  

• Quenching Template (MCP Tool Call):

  {
    "tool": "mcp_mace_run_md",
    "arguments": {
      "structure_data": "initial_structure.cif",
      "temperature": 3000.0,
      "steps": 5000,
      "timestep": 2.0,
      "ensemble": "nvt_langevin",
      "monitor": true,
      "monitor_type": "quenching",
      "monitor_params": {
        "temperature_end": 300.0,
        "steps": 5000
      },
      "output_dir": "research/quenching_output"
    }
  }
  

• Action: When a monitor triggers, the simulation stops immediately with a status: "stopped" and a clear stop_reason in the result dictionary.

2. Parameter Initialization

• Time Step:
  • 2.0 fs: Recommended for most systems without light elements (Hydrogen).
  • 0.5 - 1.0 fs: Use for systems containing Hydrogen, or at very high temperatures (> 2000K) to maintain stability.
• Thermostat: Use a coupling constant (taut) around $100 \times \text{timestep}$.
• Temperature Ramp: Start at a low temperature (50K) and ramp to the target to avoid "shock" waves from initial overlaps.

3. Handling Instability

If a simulation explodes:

1. Reduce Timestep: Try 0.5 fs.
2. Ramp Temperature: Use a slower heating rate.
3. Check Potential: Consider if the chemistry is within the training range of the foundation potential. If not, use the ml-mlip-training skill.

Examples

Monitoring stability is handled automatically by the ASE callbacks. When a monitor is triggered, the stop reason is logged to stdout and saved in the result dictionary.

Constraints

• Termination: If a monitor triggers an explosion, terminate the task and adjust parameters. Do not proceed with unstable trajectories.
• Reporting: Always report simulation parameters (ensemble, T, timestep, duration) in the research report.

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Author: Bowen Deng Contact: GitHub @learningmatter-mit