Solvers

Each solver is a backend, which wraps one implementation of one method. lightsmith backends lists them all, with licence, GPU support, differentiability and availability. You can choose a backend in three ways:

• method="auto" (the default) picks the first backend in the problem kind's preference order that is installed and accepts the problem. If none does, the error lists why each one was skipped.
• method="<method>", for example "dda", picks the first available backend for that method. It does not check whether that backend accepts the problem.
• method="<backend name>", for example "lightsmith-mie", picks exactly that implementation.

Method	Backend name(s)	Problem kinds	GPU	Gradients	Dependency
mie	miepython, lightsmith-mie	scattering	native	native (functional API)	miepython (core)
tmatrix	treams	scattering	—	—	treams (tmatrix extra)
dda	lightsmith-dda	scattering	✅	—	—
fdtd	lightsmith-fdtd	scattering, periodic, layer_stack	✅	—	—
quasistatic	lightsmith-quasistatic	scattering	—	—	—
tmm	lightsmith-tmm	layer_stack	✅	✅	—
rcwa	lightsmith-rcwa	periodic, layer_stack	✅	✅	—
fde	lightsmith-fde	mode	—	—	—
eme	lightsmith-eme	device	—	—	—
fdfd	lightsmith-fdfd	device (2D)	✅	✅ (adjoint)	—
pwe	lightsmith-pwe	bands	✅	dense path	—
circuit	lightsmith-circuit	circuit	✅	✅	—
inverse	lightsmith-inverse	optimization	✅	—	PyTorch (gpu extra)

Every backend is MIT-licensed. GPL solvers ship as separate plugin packages (see Installation).

Automatic order

Problem kind	auto tries, in order
scattering	mie → tmatrix → dda → fdtd (then any other method, such as quasistatic)
layer_stack	tmm → rcwa → fdtd
periodic	rcwa → fdtd
mode	fde
device	eme → fdfd
bands	pwe
circuit	circuit
optimization	inverse

For scattering by a single plain sphere with no field outputs, auto picks miepython. For a layered sphere, or when near or far fields are requested, it picks lightsmith-mie.

method=\"mie\" and layered spheres

method="mie" resolves to the first installed Mie backend, which is miepython, and miepython handles only homogeneous spheres. For layered spheres or field outputs, pass method="lightsmith-mie" or method="auto".

How each solver page is organised

• Method: what is actually implemented.
• Options: every key of options[<method>], with its default.
• Validation: the accuracy checks in the test suite (tests/). The tolerances are the asserted bounds; "measured" figures come from comments and docstrings in the tests.
• Limits: what the solver refuses or warns about.

GPU-only and slow tests are marked; they run with uv run pytest -m gpu and -m slow.