Why Molecular Modelers Are Embracing FIRE for Faster Geometry Optimization

If you’ve ever performed geometry optimization, you’ve likely faced the challenge of balancing speed and accuracy — especially when working with large structures or when your focus is on relaxing a system before a simulation. One of the lesser-known but highly effective methods is the FIRE Minimizer, short for Fast Inertial Relaxation Engine. It’s a powerful option available in the SAMSON platform, and it might be exactly what your workflow is missing.

The Usual Pain Point

Traditional geometry optimization methods like steepest descent often grind to a halt when molecules undergo collective or large-scale structural motions. These situations can occur during early-stage structural cleanup, post-modeling fixes, or preparing systems for further simulation. The calculations drag on, only to yield minor improvements.

Why FIRE Is Different

The FIRE algorithm, introduced by Bitzek et al. in 2006, leverages a mix of energy gradients and velocity to efficiently find geometry minima. What makes it practical for real-world modeling is how quickly it converges, especially in cases when energy changes are small but geometric rearrangements are still large.

Here are a few reasons to consider trying FIRE:

  • Significantly faster convergence than steepest descent in many scenarios.
  • Particularly good at handling collective molecular motions.
  • Works with any interaction model within SAMSON.
  • Ideal for cleanup and structural relaxation right before simulations.

Seeing Is Understanding

The two GIFs below illustrate the difference between FIRE and steepest descent when minimizing the same molecular structure. Note how quickly the FIRE algorithm overcomes large-scale motions compared to the slower and more incremental descent-based approach.

FIRE Relaxation
FIRE minimizes structural distortions quickly using inertial dynamics.
Steepest Descent Relaxation
Steepest descent moves atoms step by step, often requiring more steps overall.

Getting Started in SAMSON

Using FIRE inside SAMSON is straightforward:

  1. Log into SAMSON Connect.
  2. Find and add the FIRE Minimizer Extension.
  3. After restarting SAMSON, load your molecule and attach a simulator.
  4. In the simulator configuration, set FIRE as the state updater.

It helps to adjust the Step size and Steps according to the size and complexity of your molecular system. You can also fix the step size if you prefer more controlled dynamics. Don’t forget that if you move atoms manually while minimizing, there’s a Reset button to clear the optimization history and start fresh.

Learn more about the FIRE Minimizer in the official SAMSON tutorial.

SAMSON and all SAMSON Extensions are free for non-commercial use. You can get SAMSON at https://www.samson-connect.net.

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