error message if only one bound is inf

tidy3d/components/geometry.py

@@ -734,6 +734,17 @@ def get_center(pt_min: float, pt_max: float) -> float:
             """Returns center point based on bounds along dimension."""
             if np.isneginf(pt_min) and np.isposinf(pt_max):
                 return 0.0
+            if np.isneginf(pt_min) or np.isposinf(pt_max):
+                raise SetupError(
+                    f"Bounds of ({pt_min}, {pt_max}) supplied along one dimension. "
+                    "We currently don't support supplying a single `inf` value in bounds "
+                    "for the construction of semi-infinite `Box`. "
+                    "Instead, to construct a semi-infinite `Box`, "
+                    "please supply a large number instead of `inf`. "
+                    "For example, if the Box represents the geometry of a `Structure` in a "
+                    "`Simulation`, its bound should extend outside of the `Simulation` domain "
+                    "including PML."
+                )
             return (pt_min + pt_max) / 2.0
 
         center = tuple(get_center(pt_min, pt_max) for pt_min, pt_max in zip(rmin, rmax))