A protein structure can arrive with dozens or hundreds of crystallographic water molecules. Some may be unnecessary for the simulation, while others may be tightly bound or functionally important near an active site. Removing every water molecule is quick, but it can also discard details you intended to preserve. The useful question is not simply “Should I remove water?” but “Which water molecules should I remove?”
Before building a GROMACS simulation, a cleanup pass can prevent unwanted molecules and structural details from complicating later preparation. In SAMSON’s GROMACS Wizard, Home > Prepare includes options to remove alternate locations, ligands, water, and monatomic ions. Those broad cleanup options can be appropriate for many systems—but water removal may not suit a model where active-site or tightly bound waters matter.
A selective approach to water cleanup
Instead of deleting all waters, use the active site as a reference and select water molecules beyond a chosen distance from it. This lets you inspect the proposed selection before erasing anything. The distance is a choice for your system, not a universal cutoff: the goal is to retain waters you consider relevant and remove those outside the region you want to preserve.
- Mark the region to preserve. Select structures such as a ligand or water, residues, or atoms in the active site. The selection serves as the reference for expanding to nearby or more distant nodes.
- Open the advanced selection tools. Right-click the current selection in the Document view or Viewport, then choose Expand selection > Advanced.
- Choose water and a distance condition. In the dialog, set the node type to Water and choose beyond the distance you want to use. For example, the documentation illustration shows a selection of water beyond 5 Å; treat that as an example, not a rule for every system.
- Preview and check. Enable auto-update to see the selection as you adjust it. Before deleting, inspect the highlighted waters and confirm that the selection matches your intention.
- Erase only the reviewed selection. Right-click the selection in the Document view or Viewport and choose Erase selection.
Why previewing matters
Selection tools make it possible to target a category of nodes, but the category alone does not tell you whether a particular molecule is expendable. A water molecule near the active site may be functional, and a distance-based selection should be reviewed against the structure and the purpose of the simulation. Auto-update is useful for seeing how a distance changes the selection; the final check is still yours.
This same cautious mindset applies to the rest of preprocessing. Remove ligands only if you do not intend to simulate them. If you need a ligand or cofactor, keep it and make sure its topology can be supplied during preparation. For arbitrary molecules you want to retain, topology files such as .itp files may be generated with external tools such as ATB Server or CGenFF, then provided in the GROMACS Wizard preparation step.
For a first pass, you can load a structure such as 1AKI, clean up unwanted alternate locations and molecules, and then continue to the preparation workflow. The main benefit of selective cleanup is simple: it turns an irreversible “remove everything” choice into a reviewable decision about what belongs in your simulation model.
For the full preprocessing walkthrough and additional notes, see the GROMACS Wizard preprocessing documentation.
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