Allow groups to be passed as sequence of floats in convert_to_multigroup (#3873)

Co-authored-by: Jonathan Shimwell <drshimwell@gmail.com>
Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

Author: 3 people

openmc/model/model.py

@@ -2520,7 +2520,7 @@ def _generate_material_wise_mgxs(
     def convert_to_multigroup(
         self,
         method: str = "material_wise",
-        groups: str = "CASMO-2",
+        groups: str | Sequence[float] | openmc.mgxs.EnergyGroups = "CASMO-2",
         nparticles: int = 2000,
         overwrite_mgxs_library: bool = False,
         mgxs_path: PathLike = "mgxs.h5",
@@ -2538,9 +2538,13 @@ def convert_to_multigroup(
         ----------
         method : {"material_wise", "stochastic_slab", "infinite_medium"}, optional
             Method to generate the MGXS.
-        groups : openmc.mgxs.EnergyGroups or str, optional
-            Energy group structure for the MGXS or the name of the group
-            structure (based on keys from openmc.mgxs.GROUP_STRUCTURES).
+        groups : openmc.mgxs.EnergyGroups, str, or sequence of float, optional
+            Energy group structure for the MGXS. Can be an
+            :class:`openmc.mgxs.EnergyGroups` object, a string name of a
+            predefined group structure from :data:`openmc.mgxs.GROUP_STRUCTURES`
+            (e.g., ``"CASMO-2"``), or a sequence of floats specifying energy
+            bin boundaries in eV (e.g., ``[0.0, 1e6]`` for a single group).
+            Defaults to ``"CASMO-2"``.
         nparticles : int, optional
             Number of particles to simulate per batch when generating MGXS.
         overwrite_mgxs_library : bool, optional
@@ -2577,7 +2581,7 @@ def convert_to_multigroup(
             Valid entries for temperature_settings are the same as the valid
             entries in openmc.Settings.temperature_settings.
         """
-        if isinstance(groups, str):
+        if not isinstance(groups, openmc.mgxs.EnergyGroups):
             groups = openmc.mgxs.EnergyGroups(groups)
 
         # Do all work (including MGXS generation) in a temporary directory