CustomGridBoundaries to precisely replicate grids from a simulation o…

…f identical size

tidy3d/components/data/monitor_data.py

@@ -684,14 +684,6 @@ def dot(
         # Drop size-1 dimensions in the other data
         fields_other = {key: field.squeeze(drop=True) for key, field in fields_other.items()}
 
-        # alignment for very close coordinate
-        for key, field in fields_other.items():
-            for coord in "xyz":
-                if coord in field.dims:
-                    fields_self.update(
-                        {key: fields_self[key].reindex(**{coord: field[coord]}, method="nearest")}
-                    )
-
         # Cross products of fields
         dim1, dim2 = self._tangential_dims
         e_self_x_h_other = fields_self["E" + dim1] * fields_other["H" + dim2]

tidy3d/components/grid/grid_spec.py

@@ -199,6 +199,47 @@ def _make_coords_initial(
         return center - size / 2 + np.arange(num_cells + 1) * dl_snapped
 
 
+class CustomGridBoundaries(GridSpec1d):
+    """Custom 1D grid supplied as a list of grid cell boundary coordinates. The grid must span
+    exactly the simulation domain size along the dimension, including PML layers, symmetric regions.
+
+    Example
+    -------
+    >>> grid_1d = CustomGridCoords(boundaries=[-0.2, 0.0, 0.2, 0.4, 0.5, 0.6, 0.7])
+    """
+
+    coords: Coords1D = pd.Field(
+        ...,
+        title="Grid boundary coordinates",
+        description="Grid boundariy coordinate which must span exactly the simulation domain size, "
+        "including PML layers. It is mostly used to duplicate grids to another simulation of the same size.",
+        units=MICROMETER,
+    )
+
+    def _make_coords_initial(
+        self,
+        axis: Axis,
+        structures: List[StructureType],
+        **kwargs,
+    ) -> Coords1D:
+        """Unused. Override ``make_coords`` directly as the
+        grid coordiate for the complete simulation domain is provided already.
+        """
+
+    def make_coords(
+        self,
+        axis: Axis,
+        structures: List[StructureType],
+        symmetry: Tuple[Symmetry, Symmetry, Symmetry],
+        periodic: bool,
+        wavelength: pd.PositiveFloat,
+        num_pml_layers: Tuple[pd.NonNegativeInt, pd.NonNegativeInt],
+        snapping_points: Tuple[Coordinate, ...],
+    ) -> Coords1D:
+        """Generate 1D coords to be used as grid boundaries."""
+        return self.coords
+
+
 class CustomGrid(GridSpec1d):
     """Custom 1D grid supplied as a list of grid cell sizes centered on the simulation center.
 
@@ -454,7 +495,7 @@ def _make_coords_initial(
         return np.array(bound_coords)
 
 
-GridType = Union[UniformGrid, CustomGrid, AutoGrid]
+GridType = Union[UniformGrid, CustomGrid, AutoGrid, CustomGridBoundaries]
 
 
 class GridSpec(Tidy3dBaseModel):
@@ -657,6 +698,14 @@ def make_grid(
         coords = Coords(**coords_dict)
         return Grid(boundaries=coords)
 
+    @classmethod
+    def imported(cls, grid: Grid) -> GridSpec:
+        """Import from another simulation's grids."""
+        grid_dict = {}
+        for dim in "xyz":
+            grid_dict["grid_" + dim] = CustomGridBoundaries(coords=grid.boundaries.to_dict[dim])
+        return cls(**grid_dict)
+
     @classmethod
     def auto(
         cls,

tidy3d/components/simulation.py

@@ -37,7 +37,7 @@
 from .geometry.utils import flatten_groups, traverse_geometries
 from .geometry.utils_2d import get_bounds, get_thickened_geom, snap_coordinate_to_grid, subdivide
 from .grid.grid import Coords, Coords1D, Grid
-from .grid.grid_spec import AutoGrid, CustomGrid, GridSpec, UniformGrid
+from .grid.grid_spec import AutoGrid, GridSpec, UniformGrid
 from .lumped_element import LumpedElementType
 from .medium import (
     AbstractCustomMedium,
@@ -4260,11 +4260,7 @@ def subsection(
         elif isinstance(grid_spec, str) and grid_spec == "identical":
             # create a custom grid from existing one
             grids_1d = self.grid.boundaries.to_list
-            new_grids = [
-                CustomGrid(dl=tuple(np.diff(grids_1d[dim])), custom_offset=grids_1d[dim][0])
-                for dim in range(3)
-            ]
-            grid_spec = GridSpec(grid_x=new_grids[0], grid_y=new_grids[1], grid_z=new_grids[2])
+            grid_spec = GridSpec.imported(self.grid)
 
             # adjust region bounds to perfectly coincide with the grid
             # note, sometimes (when a box already seems to perfrecty align with the grid)