A structure relaxes, the energy changes, and the forces look reasonable—but is the model answering the question you intended to ask? For molecular modelers, a common source of wasted time is not a failed calculation; it is a calculation run with the wrong task, molecular charge, spin state, or periodic cell. UMA in SAMSON makes interactive energy and force evaluation available, but its results are most useful when the setup matches the system.
This guide focuses on a practical question: how can you use UMA for rapid structural exploration without mistaking an exploratory result for a final answer?
Start by matching the task to the system
When you create a simulator, select UMA Force Field as the interaction model. In the setup window, choose both a model and a task. The task indicates the application domain, so it is worth checking before starting:
- OMOL – Molecules is for molecular systems. It is the only task whose interface uses total charge and spin multiplicity.
- OMAT – Inorganic materials is for bulk inorganic materials workflows.
- ODAC – MOFs is for MOFs and direct-air-capture-style adsorption workflows.
- OC20 – Catalysis is for catalytic surfaces and adsorbates.
- OMC – Molecular crystals is for molecular-crystal systems.
Choosing a task is not just a menu detail: it is part of describing what kind of system you are asking UMA to evaluate. If you are unsure which model size to begin with, the practical starting point is UMA-S. The extension offers UMA-S v1.2 (2.3 GB) as the default fast workflow, UMA-S v1.1 (1.1 GB) as a lighter legacy option, and UMA-M v1.1 (11.7 GB) for cases where you want a larger model. Begin with UMA-S unless you already have a reason to choose UMA-M.
Check the details that change the request
For OMOL, make sure the total charge and spin multiplicity are consistent with the molecular system you intend to model. These values are part of the request sent to UMA, so an otherwise correct structure can still represent the wrong calculation if either setting is inappropriate.
For periodic workflows, inspect the periodic-boundary-conditions controls and the 3 × 3 cell matrix, given in angstroms. Confirm that periodic boundaries are enabled or disabled as intended and that the cell is the one you mean to use. The cell is especially important to check before interpreting results for a periodic system.
Expect a first-use setup step
Before creating the simulation, make sure the UMA Force Field and Python Scripting extensions are installed, and that Python packages can be installed on first use. UMA may prepare a dedicated Python environment and request access to the models. This initial setup can take time. Follow the instructions shown in SAMSON; after setup is complete, you may need to create the interaction model again.
Once the environment and model are ready, start the simulation using Edit > Simulate > Start. The properties window may initially report that energy is unavailable. Wait for the first UMA evaluation to finish; the total energy should appear after a successful evaluation. You can then move atoms or refine the structure while energies and forces update.
Use bond updates as exploration, not proof
The Bond update setting controls how the bond graph is handled during the simulation. Covalent recomputes covalent bonds from the current structure. The estimated Wiberg and Mayer options use a deep-learning model to estimate bond orders, while Off keeps the existing bond graph unchanged. The estimated modes can help with interactive exploration and visualization, but they are not a substitute for confirming important conclusions with an appropriate reference method.
A useful boundary for interpreting results
UMA is well suited to fast geometry refinement, exploratory relaxation, and qualitative force analysis. A sensible workflow is to use it to screen, refine, and explore structures, while treating the results as part of an investigation—not automatically as the final word. For decisions that require higher accuracy, confirm the result with a higher-accuracy reference method when needed. Starting with a small representative system can also help you check that the task, charge, spin, and periodic cell are set as intended before applying the workflow to a larger study.
For setup details and the complete tutorial, read the UMA Force Field documentation.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.