19. FAQ

This section answers a few common questions that come up repeatedly when using OpenSn.

19.1. Why is my problem split into a problem object and a solver object?

The problem object defines the transport model: mesh, materials, sources, boundaries, quadrature, and groupsets. The solver object defines how that model is advanced or converged.

This split makes it possible to keep the physical problem definition separate from the algorithm used to solve it.

19.2. Should I start with multiple groupsets?

Usually no.

For a new input, the safest starting point is a single groupset containing all groups. Once the problem is working, groupsets can be split intentionally for performance or algorithmic reasons.

If multiple groupsets are used and there is upscatter coupling across groupset boundaries, the Across-Groupset Solver (AGS) must be enabled and converged.

19.3. Why is my field function not updating?

Field functions are created from the current solution when requested. They are not continuously updated by the solvers.

If the solver state changes, either create the field function again to obtain a new snapshot of that state, or reuse the existing field function by calling Update() when CanUpdate() returns True.

19.4. How do I get normalized power or XS-weighted outputs?

Use CreateFieldFunction(name, xs_name, power_normalization_target=...).

This method can create:

• raw power with xs_name="power"

• raw XS-weighted scalar fields with built-in or custom 1D XS names

• power-normalized versions when power_normalization_target is supplied

The normalization is applied to the returned field function, not to the solver state itself.

19.5. When should I use Execute() and when should I use Advance()?

Use Execute() when the solve can run from start to finish without input changes during the run.

Use Advance() with transient problems when the input needs to update sources, boundaries, or other problem data between time steps. Advance() is designed to be used with a Python time-step loop.

19.6. What quadrature order should I use?

The quadrature must be fine enough for both the angular variation of the solution and the scattering-moment content of the problem.

If higher scattering moments are retained, the quadrature order must also be high enough to integrate those moments accurately. A quadrature that is too coarse can produce poor accuracy even if the spatial and iterative settings are otherwise reasonable.

19.7. Is single-angle aggregation always necessary when using unstructured meshes?

Yes. On unstructured meshes, angle_aggregation_type="single" should always be used.

19.8. What is the safest default sweep type?

AAH is the general default and is the safer choice for most users.

It supports the delayed-angular-flux machinery used to break intra-partition cyclic sweep dependencies.

19.9. Do I need to export field functions during every run?

No.

If an input does not need post-processed output, it can simply solve the problem and stop. Creating field functions is additional post-processing work, not part of the transport solve itself.