A familiar frustration in molecular modeling is getting a result that looks precise before you know whether the setup matches the system. With a machine-learning interaction model, the task, molecular charge, spin multiplicity, and periodic cell can all affect what you are asking it to evaluate. A useful workflow is not just about starting a simulation: it is about checking these choices before interpreting the output.
The UMA Force Field extension in SAMSON lets you evaluate energies and forces interactively while exploring or refining a structure. It is particularly useful for fast geometry refinement, exploratory relaxation, and qualitative force analysis. Treat it as a way to investigate structures and screen possibilities—not as an automatic substitute for a higher-accuracy reference method when final conclusions matter.
Start by matching the task to the system
When you add a simulator through Edit > Simulate > Add simulator and select UMA Force Field, the setup window asks you to select a model and a task. The task should describe the kind of system you are evaluating:
- OMOL for molecules
- OMAT for bulk inorganic materials
- ODAC for MOFs and direct-air-capture-style adsorption workflows
- OC20 for catalytic surfaces and adsorbates
- OMC for molecular crystals
This choice is an important first check: selecting a task that does not fit your system can make later interpretation less meaningful. For molecules using OMOL, the interface also asks for total charge and spin multiplicity. Set both to values consistent with the molecular system you intend to model; these settings are part of the request sent to UMA.
Choose a model for the first pass
The available options include UMA-S v1.2 (2.3 GB), UMA-S v1.1 (1.1 GB), and UMA-M v1.1 (11.7 GB). If you do not already have a reason to use the larger model, begin with UMA-S. This is a practical way to get a workflow running before deciding whether you need the additional capacity of UMA-M. The model files are substantial, so it is also helpful to account for download and setup time.
Before you begin, install the UMA Force Field and up-to-date Python Scripting extensions, and make sure your computer can install Python packages on first use. On the first run, UMA prepares a dedicated Python environment and may request access to the models. Follow SAMSON’s prompts; setup can take some time, and you may need to create the interaction model again once setup finishes.
Read startup behavior correctly
After the environment is ready, start the simulation using Edit > Simulate > Start. The first evaluation may take a moment. The UMA Properties window can initially report that energy is not available; that message at startup does not necessarily mean the setup has failed. Wait for the first evaluation to complete, then check whether the total energy appears. Once running, you can move atoms or refine the structure and observe updated energies and forces.
Check the details that change interpretation
For molecular systems, revisit charge and spin multiplicity if the structure or chemical state you want to investigate changes. For periodic workflows, inspect the periodic-boundary-condition setting and the 3 × 3 cell matrix, expressed in angstroms. A cell that does not represent the intended system can undermine interpretation, so check it before drawing conclusions from a periodic calculation.
The Bond update setting is another choice worth making deliberately. Covalent recomputes covalent bonds from the current structure; Off leaves the existing bond graph unchanged. Wiberg and Mayer bond-order modes provide estimates from a deep-learning model. They can help with interactive exploration, but they are estimated visualizations—not confirmation of a bonding assignment. Check important conclusions with your reference method.
A compact pre-flight checklist
- Confirm the installed extensions and allow time for first-use setup.
- Choose the task that matches the system type.
- Start with UMA-S unless you have a specific reason to choose UMA-M.
- For OMOL, verify total charge and spin multiplicity.
- For periodic systems, check the boundary-condition setting and cell matrix.
- Wait for the first evaluation before interpreting the energy display.
- Use interactive results to explore and refine; validate important conclusions with an appropriate reference method.
This sequence helps separate a setup issue from a scientific question, and makes it easier to use interactive feedback without assigning more certainty to it than it warrants. For the full setup instructions and model details, see the UMA Force Field tutorial.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.