A force-field setup can fail even when the molecular structure looks perfectly reasonable on screen. The cause is often not a single setting: missing hydrogens, an unsupported molecule, a query that selects the wrong atoms, or incompatible parameter files can all prevent parameterization. The useful first step is to treat the setup check as a diagnostic, not just a button to get past.
This workflow focuses on a common challenge: preparing a protein–ligand system in SAMSON, where the protein and ligand need different force-field treatment. It also offers a checklist for deciding what to inspect when setup validation does not succeed.
Start by identifying which atoms need parameters
Before choosing a preset, decide what your target contains. A protein, nucleic acid, water, or ions may fit a Standard workflow. A small organic molecule can also use Standard, with a suitable OpenFF option such as Sage or Parsley. For a protein–ligand complex where the protein uses Amber or CHARMM and the ligand uses OpenFF parameters, choose Composite. Use Advanced when different atom groups need their own force-field files or when you are testing custom files.
This choice matters because the setup is validated against the current target. A force field that works for the protein may not parameterize an unsupported ligand or other heterogen. If the target contains more than one chemical category, make a plan for each group rather than assuming one preset covers everything.
For a protein–ligand complex, verify the ligand selection
In Composite mode, SAMSON uses the Node Specification Language query n.c lig to identify ligand atoms. Assign the ligand atoms to the lig class, choose the protein preset and ligand force field, then select Refresh ligand query. Check the Ligand class row: its atom count should match the ligand you intended to select.
If the query selects no atoms, the ligand has not been identified as expected. If it selects every atom in the document, the query is too broad. Either result prevents the composite setup from being created. This count is a simple but important check: it helps catch a selection problem before you interpret any energy or force values.
Composite mode also requires at least one protein atom outside the ligand selection and does not support covalent bonds between protein and ligand atoms. If the ligand is covalently attached, use a force-field approach that represents that covalent chemistry directly rather than trying to work around the limitation by changing the selection.
Check structure preparation before changing force fields
For Amber or CHARMM typing, inspect the structure for common sources of failure:
- Alternate locations imported together: choose one location.
- Missing hydrogens or uncertain protonation states: decide protonation first, then add hydrogens.
- Incomplete peptide or nucleic-acid fragments: complete or cap termini as appropriate.
- Unsupported ligands, cofactors, additives, or buffer molecules: remove them if they are not part of the intended target, or parameterize them intentionally.
- Water or ions without a compatible model: choose the relevant water/ion option where available.
- Accidental covalent bonds between separate components: inspect connectivity.
In a Composite setup, do not remove a ligand that is meant to receive separate parameters. It must remain in the document and match the ligand query.
Use the status message as a checklist
The setup window checks whether the selected force field can be used with the current target. Wait for its status message before creating the interaction model. If setup fails, read the details and fix the specific issue rather than switching presets at random. In Advanced mode, for example, check whether a group query is empty or too broad, whether an earlier group already assigned its atoms, and whether a single group mixes XML and OFFXML files. Those file families belong in separate groups.
Warnings deserve attention even when setup is allowed. They may be acceptable for exploratory work, but understand them before relying on the result. If the window offers Exclude, use it only when removing those atoms from the simulator target is scientifically appropriate.
Confirm what was actually created
After creating the model and starting the simulation, inspect the Molecular Mechanics Force Fields properties. Confirm the provider and parameter set, check whether the model is constrained, and review energy, generated terms, atom types, per-atom parameters, warnings, and status. Wait for the first evaluation to finish before interpreting energy or forces.
A reliable sequence is: prepare the structure, choose the setup mode that matches its components, verify atom-group selections, resolve validation messages, and then inspect the generated model. For the full tutorial, see Molecular Mechanics Force Fields in SAMSON.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.