Automatically use periodic boundary on zero-size dimensions

CHANGELOG.md

@@ -39,6 +39,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Improved performance of `tidy3d.web.delete_old()` for large folders.
 - Upon initialization, an FDTD `Simulation` will now try to create all `ModeSolver` objects associated to `ModeSource`-s and `ModeMonitor`-s so they can be validated.
 - `tidy3d.plugins.autograd.interpolate_spline()` and `tidy3d.plugins.autograd.add_at()` can now be called with keyword arguments during tracing.
+- Zero-size dimensions automatically receive periodic boundary conditions instead of raising an error.
 
 ## [2.8.4] - 2025-05-15
 

tests/test_components/test_mode.py

@@ -151,7 +151,6 @@ def test_validation_from_simulation():
             size=(0, 5, 5),
             mode_spec=td.ModeSpec(angle_rotation=True, angle_theta=np.pi / 4),
             freqs=[td.C_0],
-            boundary_spec=td.BoundarySpec.all_sides(td.Periodic()),
         )
 
     with pytest.raises(SetupError):
@@ -160,7 +159,6 @@ def test_validation_from_simulation():
             size=(0, 5, 5),
             mode_spec=td.ModeSpec(angle_rotation=True, angle_theta=np.pi / 4),
             freqs=[td.C_0],
-            boundary_spec=td.BoundarySpec.all_sides(td.Periodic()),
         )
 
 
@@ -169,16 +167,12 @@ def get_mode_sim():
     permittivity_monitor = td.PermittivityMonitor(
         size=(1, 1, 0), center=(0, 0, 0), name="eps", freqs=FS
     )
-    boundary_spec = td.BoundarySpec(
-        x=td.Boundary.pml(), y=td.Boundary.periodic(), z=td.Boundary.pml()
-    )
     sim = td.ModeSimulation(
         size=SIZE_2D,
         freqs=FS,
         mode_spec=mode_spec,
         grid_spec=td.GridSpec.auto(wavelength=td.C_0 / FS[0]),
         monitors=[permittivity_monitor],
-        boundary_spec=boundary_spec,
     )
     return sim
 

tests/test_components/test_simulation.py

@@ -563,16 +563,19 @@ def test_validate_zero_dim_boundaries():
         pol_angle=0.0,
     )
 
-    with pytest.raises(pydantic.ValidationError):
-        td.Simulation(
-            size=(1, 1, 0),
-            run_time=1e-12,
-            sources=[src],
-            boundary_spec=td.BoundarySpec(
-                x=td.Boundary.periodic(),
-                y=td.Boundary.periodic(),
-                z=td.Boundary.pml(),
-            ),
+    with AssertLogLevel("WARNING", contains_str="Periodic"):
+        assert (
+            td.Simulation(
+                size=(1, 1, 0),
+                run_time=1e-12,
+                sources=[src],
+                boundary_spec=td.BoundarySpec(
+                    x=td.Boundary.periodic(),
+                    y=td.Boundary.periodic(),
+                    z=td.Boundary.pml(),
+                ),
+            ).boundary_spec.z
+            == td.Boundary.periodic()
         )
 
     # zero-dim simulation with an absorbing boundary any other direction should not error

tidy3d/components/eme/simulation.py

@@ -1183,4 +1183,4 @@ def _cell_index_pairs(self) -> list[pd.NonNegativeInt]:
             pairs = set(self.eme_grid_spec._cell_index_pairs)
         return list(pairs)
 
-    _boundaries_for_zero_dims = validate_boundaries_for_zero_dims()
+    _boundaries_for_zero_dims = validate_boundaries_for_zero_dims(warn_on_change=False)

tidy3d/components/mode/simulation.py

@@ -8,7 +8,7 @@
 import pydantic.v1 as pd
 
 from tidy3d.components.base import cached_property
-from tidy3d.components.boundary import Boundary, BoundarySpec
+from tidy3d.components.boundary import BoundarySpec
 from tidy3d.components.geometry.base import Box
 from tidy3d.components.grid.grid import Grid
 from tidy3d.components.grid.grid_spec import GridSpec
@@ -213,18 +213,6 @@ def plane_in_sim_bounds(cls, val, values):
             raise SetupError("'ModeSimulation.plane' must intersect 'ModeSimulation.geometry.")
         return val
 
-    @pd.validator("boundary_spec", always=True)
-    def boundaries_for_zero_dims(cls, val, values):
-        """Replace with periodic boundary along zero-size dimensions."""
-        boundaries = [val.x, val.y, val.z]
-        size = values.get("size")
-
-        for dim, size_dim in enumerate(size):
-            if size_dim == 0:
-                boundaries[dim] = Boundary.periodic()
-
-        return BoundarySpec(x=boundaries[0], y=boundaries[1], z=boundaries[2])
-
     def _post_init_validators(self) -> None:
         """Call validators taking `self` that get run after init."""
         ModeSolver._validate_mode_plane_radius(
@@ -524,4 +512,4 @@ def plot_pml_mode_plane(
     def validate_pre_upload(self, source_required: bool = False):
         self._mode_solver.validate_pre_upload(source_required=source_required)
 
-    _boundaries_for_zero_dims = validate_boundaries_for_zero_dims()
+    _boundaries_for_zero_dims = validate_boundaries_for_zero_dims(warn_on_change=False)

tidy3d/components/simulation.py

@@ -173,7 +173,7 @@
 RF_FREQ_WARNING = 300e9
 
 
-def validate_boundaries_for_zero_dims():
+def validate_boundaries_for_zero_dims(warn_on_change: bool = True):
     """Error if absorbing boundaries, bloch boundaries, unmatching pec/pmc, or symmetry is used along a zero dimension."""
 
     @pydantic.validator("boundary_spec", allow_reuse=True, always=True)
@@ -192,11 +192,15 @@ def boundaries_for_zero_dims(cls, val, values):
                 num_bloch_bdries = sum(isinstance(bnd, BlochBoundary) for bnd in boundary)
 
                 if num_absorbing_bdries > 0:
-                    raise SetupError(
-                        f"The simulation has zero size along the {axis} axis, so "
-                        "using a PML or absorbing boundary along that axis is incorrect. "
-                        f"Use either 'Periodic' or 'BlochBoundary' along {axis}."
-                    )
+                    pbc = Boundary(minus=Periodic(), plus=Periodic())
+                    val = val.updated_copy(**{axis: pbc})
+                    if warn_on_change:
+                        log.warning(
+                            f"The simulation has zero size along the {axis} axis, so "
+                            "using a PML or absorbing boundary along that axis is incorrect. "
+                            f"Use either 'Periodic' or 'BlochBoundary' along {axis}. "
+                            "Using 'Periodic' boundary by default."
+                        )
 
                 if num_bloch_bdries > 0:
                     raise SetupError(