Why FIRE Minimization Outpaces Steepest Descent (and When to Use It)

As a molecular modeler, you’ve likely faced slow structural relaxation in systems with collective motions—especially when preparing simulations or refining models after manual manipulations. If you’ve used the steepest descent algorithm and watched the energy barely drop while your patience vanishes, there’s a faster alternative that could save you time and frustration: the FIRE minimizer.

FIRE, short for Fast Inertial Relaxation Engine, is an efficient geometry optimization algorithm implemented in SAMSON. It’s designed to accelerate convergence, particularly in situations where geometry changes are large but energy gradients are small—cases where steepest descent can struggle.

What Makes FIRE Faster?

The key advantage of FIRE lies in how it uses both gradient and velocity directions to adaptively steer atoms. In contrast, steepest descent simply follows the gradient direction, which becomes inefficient when potential energy surfaces are relatively flat.

FIRE Relaxation — FIRE Minimizer: Fast convergence even during large-scale motion

Steepest Descent Relaxation — Steepest Descent: Slower to converge on flat regions

When to Prefer FIRE Over Steepest Descent

Collective motions: When entire domains or large parts of molecules need to shift or relax, FIRE is generally much faster.
Flat energy landscapes: In regions where energy barely changes but the geometry still needs adjustment, FIRE can reach minima without wasting steps.
Pre-simulation optimizations: Before launching MD or QM simulations, FIRE helps create well-relaxed starting structures, minimizing bad contacts or artificial strains.

Tips for Using FIRE Effectively

To better observe progress, increase the Steps parameter so that updates in the viewport are more spaced out but more noticeable.
If you manually move atoms mid-optimization, use the Reset button to clear the FIRE internal history and avoid convergence issues.
FIRE is available for any interaction model in SAMSON—simply add it as a State Updater when adding a simulator.

More Than a Standalone Tool

FIRE is also integrated in more advanced workflows. For example, it’s used in the Molecular Restrainer Extension for refining structures derived from NMR experiments. Its seamless integration across different parts of SAMSON makes it a versatile tool in structure-based research and drug design pipelines.

If you’re still using steepest descent for every structure, consider giving FIRE a try—especially when your molecules are slow to settle. It might save you quite a few iterations.

To learn more, visit the full tutorial at https://documentation.samson-connect.net/tutorials/fire/ready-set-fire/.

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