A force-field setup can fail before a simulation even begins—or, more subtly, appear to work while the atoms you intended to parameterize are not the atoms the simulator actually selected. These problems are often caused not by the energy calculation, but by a mismatch between the molecular structure, the force-field files, and the target selection.
A reliable approach is to treat setup as a short preflight check: prepare the structure, choose the workflow that fits the chemistry, confirm which atoms are included, and read the validation status before creating the interaction model. Here is a practical way to do that in SAMSON.
1. Start with the system, not the force-field menu
First identify what you are parameterizing. A protein, nucleic acid, water, or ions generally points to a Standard Amber or CHARMM setup. A small organic molecule can use a Standard Sage or Parsley setup. For a protein–ligand complex where the ligand needs OpenFF parameters, use Composite: one force field for the protein and another for the ligand. Choose Advanced when different atom groups need separately assigned files, or when you need custom XML or OFFXML files.
This decision matters because each workflow solves a different assignment problem. Standard is for one force-field setup; Composite separates a protein from a ligand; Advanced gives you explicit control over groups and files.
2. Clean up biomolecules before typing
Amber and CHARMM typing can be affected by structural details that are easy to overlook. Before setup, check alternate locations, hydrogens and protonation states, terminal residues, and unsupported ligands, cofactors, additives, or buffer molecules. Also check that waters and ions have a compatible model and that the structure has no accidental covalent bonds between otherwise separate components.
For example, importing multiple alternate locations together can create an ambiguous target. Missing hydrogens or incomplete termini can also prevent a force field from assigning parameters. Preparation is not merely cosmetic: it helps ensure that the chemical structure presented to the parameterizer is the one you intend to model.
3. Make selections testable
Composite setup depends on the ligand selection. Assign the ligand atoms to the NSL class lig, then refresh the ligand query and check the reported atom count for n.c lig. The query should select the ligand—and only the ligand. If it selects zero atoms or every atom in the document, the composite setup cannot be created.
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, do not try to solve that mismatch by changing the selection; use a force field that represents the covalent chemistry directly.
4. Read validation as diagnostic information
Wait for the setup status before pressing OK. SAMSON checks whether the chosen force field can parameterize the current target, and the status message can point to the issue. Common causes include unsupported molecules in an Amber- or CHARMM-only setup, an incorrect ligand query, an empty or overly broad Advanced group, or XML and OFFXML files mixed within one group.
In Advanced mode, explicit groups are evaluated in their displayed order; earlier groups have priority, and the default * group handles atoms not matched earlier. This means group order is part of the assignment logic. Check diagnostics for groups that match no atoms or whose atoms have already been assigned to a higher-priority group. Keep XML and OFFXML files in separate groups.
A warning is not automatically a reason to stop, but it is a reason to pause. Use Exclude only when removing the affected atoms from the simulator target is scientifically appropriate—not simply to make setup pass.
5. Confirm what was actually created
Once the interaction model is ready, start the simulation and wait for the first energy and force evaluation. Then inspect the properties window. Confirm the provider and parameter set, check whether constraints are enabled, and review energy and generated terms. Under Details, inspect atom types, per-atom parameters, warnings, and status. These checks help distinguish a successfully created model from a scientifically appropriate one.
For routine interactive work, the constrained option is generally the practical starting point; use unconstrained mode when preserving X–H bond flexibility is specifically needed. In all cases, interpret energies and forces only after reviewing setup warnings and confirming the assigned parameters.
For the full workflow and interface details, read the Molecular Mechanics Force Fields tutorial.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.