How to Minimize a Specific Part of a Molecule

For molecular modelers, achieving precision in structural refinement can be a recurring challenge, particularly when focusing on specific parts of a molecule while leaving other areas untouched. SAMSON, the integrative molecular design platform, provides an effective solution for such cases through a freezing technique during minimization. This blog post explains how to minimize only a selected part of a molecule by freezing other parts, offering clarity for enhancing your workflow.

Why Would You Want to Minimize Only Part of a Molecule?

When working with large molecules or structures, it is often unnecessary—or even counterproductive—to minimize the entire system every time. Imagine tweaking atomic positions in a small region of a protein or adjusting a binding site in a ligand-receptor complex. You may want the rest of the molecule to remain unaffected during these refinements. SAMSON’s freezing and unfreezing feature is built precisely for such use cases.

How Does It Work?

To minimize a specific part of a molecule in SAMSON, follow these steps:

Select the entire molecule or leave the selection empty if you intend to specify which atoms to freeze later. You can find more details about selection in Selecting.
Navigate to Edit > Freeze to freeze the currently selected atoms or the whole molecule if your selection is empty. The frozen atoms will remain fixed during minimization.
Select the specific atoms or region of the molecule that you want to minimize.
Use Edit > Unfreeze to unfreeze only the selected atoms which you want to minimize (leaving the rest frozen).
Start the minimization process by clicking Edit > Minimize. You can stop it at any time using the same command.
Finally, once the process is complete, you can unfreeze the rest of the molecule (if desired) by either selecting the entire molecule or leaving the selection empty, and using Edit > Unfreeze.

During this process, frozen atoms are visually distinguishable by a dark blue overlay in the viewport, making it easy to identify which parts of the structure are fixed.

Practical Application Example

Let’s say you are working on a protein-ligand interaction model and only want to refine the positions of atoms in the active site without disturbing the overall protein structure. You can:

Freeze the entire molecule.
Unfreeze only the active site atoms.
Perform minimization to optimize the active site structure for improved interaction accuracy.

Any atoms outside the minimized selection will remain in their original configuration, while the active site reaches an optimized geometry.

To Visualize the Process

Here is an image showing the interactive minimization preferences in SAMSON, which can help you customize your settings:

Ready to Improve Your Molecular Models?

This approach saves time and computational resources, making it an efficient way to focus your efforts on the parts of a molecule that truly matter. To dive deeper into SAMSON’s minimization features, visit the full documentation at https://documentation.samson-connect.net/users/latest/minimizing/.

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