A protein–ligand complex can look perfectly assembled in a molecular viewer and still fail during simulation setup. A common source of trouble is a mismatch between the ligand in the structure and the ligand described by its topology: names may differ, hydrogens may be missing, or the topology may depend on a force field that has not been supplied.
This guide focuses on avoiding those handoff problems when preparing a non-covalent protein–ligand complex with SAMSON’s GROMACS Wizard. The key idea is to treat the ligand’s coordinates, atom names, topology, and force-field requirements as parts of one coordinated workflow—not as separate files to assemble at the last minute.
1. Start with a compatible parameterization
If the ligand is not already represented as a standard residue in the force field you plan to use, it needs parameters. Choose a parameterization tool or server that supports that force field. Examples include Antechamber for AMBER, ATB for GROMOS96 54A7, CGenFF for CHARMM, and LigParGen for OPLS-AA.
Before exporting the ligand from SAMSON, check which input formats your selected service accepts. This small check can prevent a round trip through unnecessary file conversions. Also confirm that the ligand has the required hydrogens: many parameterization tools expect them to be present and correctly specified.
In SAMSON, hydrogens can be added through Edit > Add hydrogens. For ligands found in the Chemical Component Dictionary (CCD), SAMSON can use the CCD’s hydrogen names. For ligands absent from the CCD, hydrogen placement is based on valences, so aromaticity and charges should already be represented correctly. If a non-standard ligand file contains this chemical information—for example, a .mol2 file—use that source when adding hydrogens.
2. Export the right ligand, then check what comes back
When the ligand is part of a larger complex, select it in the Document view and choose Home > File > Save selection as…. Save it in a format accepted by the parameterization workflow. If you already have a ligand file with hydrogens, you can use that directly. SAMSON can also convert formats by loading a file and saving it in the required format.
After parameterization, the central deliverable is usually an include topology file, .itp. Some services also provide an updated structure and, in certain cases, a force field. Keep these outputs together. The updated structure is useful for checking whether the service changed atom names, added hydrogens, or otherwise changed the ligand representation.
Practical checkpoint: before continuing, compare the returned ligand with the ligand you intend to simulate. If atom names or hydrogens changed, use the updated representation in the complex; do not assume the original coordinates and the new topology still correspond atom-for-atom.
3. Put the parametrized ligand into the complex
If the existing ligand has not changed, you may be able to keep the current protein–ligand structure. If it has changed, position the updated ligand where the original ligand was and remove the old copy. If you do not yet have a ready-to-use complex, you can position the ligand with SAMSON’s move tools, dock it using AutoDock Vina Extended, or append its structure to the protein structure file. When appending, make sure the structures do not overlap and are assigned separate chains or consecutive sequence numbering.
One detail deserves a deliberate check: the ligand name in the structure must match the name used in the topology. A mismatch here can prevent the structure and topology from being associated as intended.
4. Add the topology—and the force field it needs
Open GROMACS Wizard at Home > Apps > Biology > GROMACS Wizard, choose the Prepare tab, select the protein–ligand system in Document view, and click Set system.
In the Choose model section, use Edit beside the include topology files (.itp) option and browse to the ligand topology file. If the parameterization service supplied a custom force field that is not included by default, add it using the force-field Add button. For example, ATB may provide gromos54a7_atb. GROMACS Wizard stores an added force field for future use; removing custom force fields removes the added custom entries, not the standard ones.
5. Prepare and inspect the result
Complete the remaining preparation choices, such as solvent model, periodic boundary box, and ions, then click Prepare. Read any warnings that appear; they may flag details relevant to the selected force field. To inspect the run log, open Output. Once preparation succeeds, the result is saved in the results folder, and you can optionally load it into SAMSON to check the prepared system.
This workflow covers non-covalently linked complexes. A covalent protein–ligand link needs special treatment of the linking bond during parameterization, so it should not be treated as an ordinary ligand topology handoff.
For the complete walkthrough and related preparation steps, read the original protein–ligand systems tutorial.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.