Why is a simulation diverging?

Date	Category
2023-10-24 15:55:24	Simulation Troubleshoot

Sometimes, a simulation is numerically unstable and can result in divergence. All known cases where this may happen are related to PML boundaries and/or dispersive media. Below is a checklist of things to consider.

• For dispersive materials with \(\epsilon_{\infty} < 1\), decrease the value of the Courant stability factor to below \(\sqrt{\epsilon_{\infty}}\).

• Move PML boundaries further away from structure interfaces inside the simulation domain, or from sources that may be injecting evanescent waves, like PointDipole, [UniformCurrentSource](https://docs.flexcompute.com/projects/tidy3d/en/latest/api/_autosummary/tidy3d.UniformCurrentSource.html#tidy3d UniformCurrentSource), or CustomFieldSource.

• Make sure structures are translationally invariant into the PML, or if not possible, use Absorber boundaries.

• Remove dispersive materials extending into the PML, or if not possible, use Absorber boundaries.

• If using our fitter to fit your own material data, ensure you are using the plugins.StableDispersionFitter.

• If none of the above work, try using StablePML or Absorber boundaries anyway (note: these may introduce more reflections than in usual simulations with regular PML).