Molecular modelers often face challenges when preparing systems for GROMACS simulations. A common pain point is dealing with unnecessary or problematic data that can slow down simulations or lead to errors during setup. Cleaning your system before simulations is essential to ensure efficiency and accuracy. In this blog post, we’ll guide you through the process of cleaning molecular systems using SAMSON’s GROMACS Wizard.
Why Clean Your System?
Simulation models can often become cluttered with alternate locations, extraneous water molecules, unwanted ligands, or monatomic ions. If left unaddressed, these can cause unnecessary complexity, compromise the accuracy of simulations, or even create issues during topology generation. Pre-processing the system is a vital step to eliminate such potential hurdles and start with a clean, simulation-ready setup.
Step-by-Step: Cleaning Your Molecular Model
Here is how you can efficiently clean your system using SAMSON:
1. Load Your System
First, load the structure of your molecular system into SAMSON:
- Open Home > Fetch in the menu.
- Enter
1AKI(or another PDB ID of your choice) into the PDB input field. - Click the Load button to import the structure. If prompted with import parameters, the default settings generally work fine.
If you already have a local file, you can load it using Home > Open (Ctrl/Cmd + O).
2. Prepare the system
Once your molecular structure is loaded, proceed with system cleaning using Home > Prepare. This step covers key tasks like:
- Removing alternate locations: Alternate locations can create ambiguity. Ensure that all molecules have a single, clear position.
- Eliminating unnecessary ligands: Small molecules that you do not wish to simulate can be removed. Carefully verify your selection to avoid removing functional ligands or cofactors.
- Removing water: Bulk water molecules can often be removed to reduce simulation overhead. However, be mindful of functional waters, such as those in active sites, that may need to be retained. For precise control, see the section on deleting crystal waters below.
- Excluding monatomic ions: If ions are not critical to your simulation, filter them out for a cleaner structure.
Optionally, you can add hydrogens to the system during this step, though GROMACS can perform this task later as well.
3. Deleting Crystal Waters Outside the Active Site
In some cases, you’ll want to preserve specific water molecules—such as those bound tightly to the active site—while removing others. Here’s how you can do that in SAMSON:
- Select the structures, residues, or atoms involved in the active site.
- Right-click on the selection in the Document view or Viewport and choose Expand selection > Advanced.
- In the dialog, filter by Water node type and set a distance beyond which water molecules should be excluded. Use the auto-update option to preview your selection.
- Click OK and verify the selection. Then, right-click and choose Erase selection.
Benefits of Pre-Processing
After completing these steps, your molecular system will be cleaner and better prepared for simulation tasks. This process helps avoid common pitfalls and ensures that the system contains only the essential components needed for accurate modeling.
To dive even deeper, visit the full tutorial at SAMSON’s documentation on system pre-processing. It contains additional details and related resources to ensure a smooth experience preparing your simulation model.
SAMSON and all SAMSON Extensions are free for non-commercial use. Get started by downloading SAMSON at https://www.samson-connect.net.