A molecular structure can look reasonable on screen and still be a poor starting point for an energy calculation. The selected model task may not match the system, a molecule’s charge or spin may be wrong, or a periodic cell may need checking. These details are easy to overlook when your main goal is simply to move atoms and see how the structure responds.
UMA Force Field in SAMSON lets you evaluate energies and forces interactively. The key is to treat setup as part of the modeling—not as a form to click through. This short workflow helps you get useful feedback while keeping the limits of that feedback in view.
1. Match the task to the system
When you create the interaction model, choose the UMA task that fits the kind of structure you are evaluating:
- OMOL – Molecules: molecular systems. This is the task whose interface uses total charge and spin multiplicity.
- OMAT – Inorganic materials: bulk inorganic materials workflows.
- ODAC – MOFs: MOFs and direct-air-capture-style adsorption workflows.
- OC20 – Catalysis: catalytic surfaces and adsorbates.
- OMC – Molecular crystals: molecular-crystal systems.
A useful first question is not “Which task sounds most advanced?” but “What kind of system am I actually asking UMA to evaluate?” Choosing by system type gives the calculation a more appropriate starting point.
2. Start with a practical model choice
The available choices include UMA-S (2.3 GB, v1.2), UMA-S (1.1 GB, v1.1), and UMA-M (11.7 GB, v1.1). For a first interactive workflow, the documentation recommends starting with UMA-S unless you already know you need the larger UMA-M model. The lighter UMA-S v1.1 option is also available as a legacy choice.
Before you begin, check that the UMA Force Field and Python Scripting extensions are installed, and that your machine can install Python packages on first use. The first setup may take time: SAMSON prepares a dedicated Python environment and may request access to UMA models. Follow the prompts, and be prepared to create the interaction model again after setup completes.
3. Check the molecular inputs that change the request
For OMOL, confirm the molecule’s total charge and spin multiplicity in the UMA Properties window. These values are part of the request sent to UMA, so use values consistent with the system you intend to model. If you are unsure what they should be, resolve that question before interpreting the energy or forces.
For periodic workflows, check whether periodic boundary conditions are enabled and inspect the 3 × 3 cell matrix in angstroms. A cell is part of the setup, not just a visual boundary around the atoms. Verify it before drawing conclusions from a periodic calculation.
4. Know what the bond display means
The Bond update control can recompute covalent bonds, show estimated Wiberg or Mayer bond orders, or leave the existing bond graph unchanged. The Wiberg and Mayer options are estimated bond-order visualizations. They can help with interactive exploration, but they are not a substitute for checking important conclusions with your reference method.
This distinction is useful when a structure changes during relaxation: an informative-looking bond display is still an estimate. Treat it as a way to inspect the evolving structure, not as final proof of a bonding assignment.
5. Run a small test before scaling up
Once the environment is ready and the model is accessible, start the simulation using Edit > Simulate > Start. The energy may initially be unavailable while the first UMA evaluation runs. Wait for that evaluation to complete before judging the result; the total energy appears in the UMA Properties window after a successful evaluation. Then try moving atoms or refining the structure and observe how the energy and forces update.
For a dependable routine, test a small representative system first. Confirm the task, model, charge and spin where relevant, and periodic cell where relevant. Use UMA for rapid screening, exploratory relaxation, and qualitative force analysis; when final accuracy matters, confirm the result with an appropriate higher-accuracy reference method.
Read the complete UMA Force Field tutorial for the setup steps and interface details.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.