avoid need to set particles and batches for dagmc models when calling convert_to_multigroup (#3801)

Co-authored-by: Jon Shimwell <jon@proximafusion.com>
Co-authored-by: Patrick Shriwise <pshriwise@gmail.com>
Co-authored-by: GuySten <62616591+GuySten@users.noreply.github.com>

openmc/model/model.py

@@ -2584,9 +2584,16 @@ class Model:
             # TODO: Can this be done without having to init/finalize?
             for univ in self.geometry.get_all_universes().values():
                 if isinstance(univ, openmc.DAGMCUniverse):
+                    # Initialize in stochastic volume mode (non-transport mode)
+                    # This mode doesn't require
+                    # valid transport settings like particles/batches
+                    original_run_mode = self.settings.run_mode
+                    self.settings.run_mode = 'volume' 
                     self.init_lib(directory=tmpdir)
                     self.sync_dagmc_universes()
                     self.finalize_lib()
+                    # Restore original run mode
+                    self.settings.run_mode = original_run_mode
                     break
 
             # Make sure all materials have a name, and that the name is a valid HDF5

tests/unit_tests/dagmc/test_convert_to_multigroup.py

@@ -0,0 +1,53 @@
+"""Test that convert_to_multigroup works with DAGMC models without requiring
+particles/batches to be set beforehand.
+"""
+
+from pathlib import Path
+import pytest
+import openmc
+import openmc.lib
+
+pytestmark = pytest.mark.skipif(
+    not openmc.lib._dagmc_enabled(),
+    reason="DAGMC CAD geometry is not enabled.")
+
+
+def test_convert_to_multigroup_without_particles_batches(run_in_tmpdir):
+    """Test that convert_to_multigroup works with DAGMC model without
+    setting particles/batches beforehand."""
+    openmc.reset_auto_ids()
+    
+    mat = openmc.Material(name="mat")
+    mat.add_nuclide("Fe56", 1.0)
+    mat.set_density("g/cm3", 7.0)
+
+    # Use minimal tetrahedral DAGMC file
+    dagmc_file = Path(__file__).parent / "dagmc_tetrahedral_no_graveyard.h5m"
+    dagmc_univ = openmc.DAGMCUniverse(dagmc_file, auto_geom_ids=True)
+    bound_dagmc_univ = dagmc_univ.bounded_universe(padding_distance=1)
+
+    # Create model WITHOUT setting particles or batches
+    model = openmc.Model()
+    model.materials = openmc.Materials([mat])
+    model.geometry = openmc.Geometry(bound_dagmc_univ)
+    model.settings = openmc.Settings()  # Note: no particles or batches set!
+
+    model.settings.run_mode = 'fixed source'
+    
+    # Create a point source
+    my_source = openmc.IndependentSource()
+    my_source.space = openmc.stats.Point((0.25, 0.25, 0.25))
+    my_source.energy = openmc.stats.delta_function(14e6)
+    model.settings.source = my_source
+
+    # This should work without requiring particles/batches to be set
+    # convert_to_multigroup handles initialization internally using non-transport mode
+    model.convert_to_multigroup(
+        method='material_wise',
+        groups='CASMO-2',
+        nparticles=10,
+        overwrite_mgxs_library=True
+    )
+
+    # Verify the model was converted successfully
+    assert model.settings.energy_mode == 'multi-group'