Add density and volume override support to DAGMC cell spec

Density overrides:
- Add parse_cell_density_xml() free function (cell.h/cell.cpp), used by
  both CSGCell::CSGCell (replacing inline parsing) and DAGUniverse XML
  constructor
- Add DensityOverrides type alias and thread it through initialize() and
  init_geometry() in DAGUniverse; apply density_mult_ in init_geometry()
  using the same deferred-multiplier convention as CSG cells
- Remove density from the forbidden-attribute block in the DAGMC XML
  constructor and from DAGMCCell validation

Volume overrides:
- Volume is Python-only (not stored in C++ Cell); no C++ changes needed
- Remove volume from forbidden-attribute checks in DAGMCCell
- Add a Notes section to DAGMCCell docstring warning that manually
  specifying volume may be inconsistent with DAGMC's triangulated-surface
  geometry, which can support exact volume computation

translation and rotation remain unsupported and are rejected explicitly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: pshriwise

include/openmc/cell.h

@@ -162,6 +162,13 @@ vector<int32_t> parse_cell_material_xml(pugi::xml_node node, int32_t cell_id);
 //! \return Vector of temperatures in Kelvin
 vector<double> parse_cell_temperature_xml(pugi::xml_node node, int32_t cell_id);
 
+//! Parse densities (in g/cm³) from a <cell> XML node.
+//! Validates that all values are positive and the list is non-empty.
+//! \param node XML node containing a "density" attribute or child element
+//! \param cell_id Cell ID used in error messages
+//! \return Vector of densities in g/cm³
+vector<double> parse_cell_density_xml(pugi::xml_node node, int32_t cell_id);
+
 //==============================================================================
 
 class Cell {

include/openmc/dagmc.h

@@ -96,6 +96,7 @@ class DAGUniverse : public Universe {
 public:
   using MaterialOverrides = std::unordered_map<int32_t, vector<int32_t>>;
   using TemperatureOverrides = std::unordered_map<int32_t, vector<double>>;
+  using DensityOverrides = std::unordered_map<int32_t, vector<double>>;
 
   explicit DAGUniverse(pugi::xml_node node);
 
@@ -116,7 +117,8 @@ class DAGUniverse : public Universe {
   //! assignments, etc.
   void initialize();
   void initialize(const MaterialOverrides& material_overrides,
-    const TemperatureOverrides& temperature_overrides);
+    const TemperatureOverrides& temperature_overrides,
+    const DensityOverrides& density_overrides = {});
 
   //! Reads UWUW materials and returns an ID map
   void read_uwuw_materials();
@@ -194,7 +196,8 @@ class DAGUniverse : public Universe {
   void init_dagmc();    //!< Create and initialise DAGMC pointer
   void init_metadata(); //!< Create and initialise dagmcMetaData pointer
   void init_geometry(const MaterialOverrides& material_overrides,
-    const TemperatureOverrides& temperature_overrides);
+    const TemperatureOverrides& temperature_overrides,
+    const DensityOverrides& density_overrides);
 
   std::string
     filename_; //!< Name of the DAGMC file used to create this universe

openmc/dagmc.py

@@ -617,6 +617,14 @@ class DAGMCCell(openmc.Cell):
     DAG_parent_universe : int
         The parent universe of the cell.
 
+    Notes
+    -----
+    DAGMC geometries are composed of triangulated surfaces, which means cell
+    volumes can in principle be computed exactly (e.g. via mesh-based
+    integration). Manually specifying :attr:`volume` overrides any such
+    calculation and may introduce inconsistencies if the value does not
+    accurately reflect the true geometric volume.
+
     """
     def __init__(self, cell_id=None, name='', fill=None):
         super().__init__(cell_id, name, fill, None)
@@ -656,11 +664,16 @@ def create_xml_subelement(self, xml_element, memo=None):
         ):
             raise TypeError("DAGMC cell temperature overrides require a "
                             "material fill.")
+        if self.density is not None and self.fill_type not in (
+            'material', 'distribmat'
+        ):
+            raise TypeError("DAGMC cell density overrides require a "
+                            "material fill.")
         if any(getattr(self, attr) is not None for attr in (
-            'density', 'translation', 'rotation', 'volume'
+            'translation', 'rotation'
         )):
-            raise TypeError("DAGMC cell overrides currently only support "
-                            "material fills.")
+            raise TypeError("DAGMC cell overrides do not support translation "
+                            "or rotation.")
         return super().create_xml_subelement(xml_element, memo)
 
     @classmethod
@@ -687,11 +700,11 @@ def from_xml_element(cls, elem, mats):
             if get_text(elem, tag) is not None:
                 raise ValueError(
                     f"DAGMC cell {cell_id} override cannot specify '{tag}'.")
-        for tag in ('density', 'translation', 'rotation', 'volume'):
+        for tag in ('translation', 'rotation'):
             if get_text(elem, tag) is not None:
                 raise ValueError(
-                    f"DAGMC cell {cell_id} override currently only supports "
-                    f"material fills (found unsupported '{tag}').")
+                    f"DAGMC cell {cell_id} override does not support "
+                    f"'{tag}'.")
         if get_elem_list(elem, 'material', str) is None:
             raise ValueError(
                 f"DAGMC cell {cell_id} must specify a material override.")

src/cell.cpp

@@ -379,6 +379,23 @@ vector<double> parse_cell_temperature_xml(pugi::xml_node node, int32_t cell_id)
   return temperatures;
 }
 
+vector<double> parse_cell_density_xml(pugi::xml_node node, int32_t cell_id)
+{
+  auto densities = get_node_array<double>(node, "density");
+  if (densities.empty()) {
+    fatal_error(fmt::format(
+      "An empty density element was specified for cell {}", cell_id));
+  }
+  for (auto rho : densities) {
+    if (rho <= 0) {
+      fatal_error(fmt::format(
+        "Cell {} was specified with a density less than or equal to zero",
+        cell_id));
+    }
+  }
+  return densities;
+}
+
 //==============================================================================
 // CSGCell implementation
 //==============================================================================
@@ -457,7 +474,7 @@ CSGCell::CSGCell(pugi::xml_node cell_node)
   // Note: calculating the actual density multiplier is deferred until materials
   // are finalized. density_mult_ contains the true density in the meantime.
   if (check_for_node(cell_node, "density")) {
-    density_mult_ = get_node_array<double>(cell_node, "density");
+    density_mult_ = parse_cell_density_xml(cell_node, id_);
     density_mult_.shrink_to_fit();
 
     // Make sure this is a material-filled cell.
@@ -478,15 +495,6 @@ CSGCell::CSGCell(pugi::xml_node cell_node)
           id_));
       }
     }
-
-    // Make sure all densities are non-negative and greater than zero.
-    for (auto rho : density_mult_) {
-      if (rho <= 0) {
-        fatal_error(fmt::format(
-          "Cell {} was specified with a density less than or equal to zero",
-          id_));
-      }
-    }
   }
 
   // Read the region specification.

src/dagmc.cpp

@@ -52,6 +52,7 @@ DAGUniverse::DAGUniverse(pugi::xml_node node)
 {
   MaterialOverrides material_overrides;
   TemperatureOverrides temperature_overrides;
+  DensityOverrides density_overrides;
 
   if (check_for_node(node, "id")) {
     id_ = std::stoi(get_node_value(node, "id"));
@@ -102,12 +103,10 @@ DAGUniverse::DAGUniverse(pugi::xml_node node)
         fatal_error(fmt::format(
           "DAGMC cell {} override cannot specify a universe.", cell_id));
       }
-      if (check_for_node(cell_node, "density") ||
-          check_for_node(cell_node, "translation") ||
-          check_for_node(cell_node, "rotation") ||
-          check_for_node(cell_node, "volume")) {
+      if (check_for_node(cell_node, "translation") ||
+          check_for_node(cell_node, "rotation")) {
         fatal_error(fmt::format(
-          "DAGMC cell {} override currently only supports material fills.",
+          "DAGMC cell {} override does not support translation or rotation.",
           cell_id));
       }
       if (!check_for_node(cell_node, "material")) {
@@ -126,14 +125,19 @@ DAGUniverse::DAGUniverse(pugi::xml_node node)
         temperature_overrides.emplace(
           cell_id, parse_cell_temperature_xml(cell_node, cell_id));
       }
+
+      if (check_for_node(cell_node, "density")) {
+        density_overrides.emplace(
+          cell_id, parse_cell_density_xml(cell_node, cell_id));
+      }
     }
   } else if (check_for_node(node, "material_overrides")) {
     fatal_error(
       "DAGMCUniverse <material_overrides> is no longer supported. Use nested "
       "<cell> elements under <dagmc_universe> instead.");
   }
 
-  initialize(material_overrides, temperature_overrides);
+  initialize(material_overrides, temperature_overrides, density_overrides);
 }
 
 DAGUniverse::DAGUniverse(
@@ -152,9 +156,10 @@ DAGUniverse::DAGUniverse(std::shared_ptr<moab::DagMC> dagmc_ptr,
 {
   MaterialOverrides material_overrides;
   TemperatureOverrides temperature_overrides;
+  DensityOverrides density_overrides;
   set_id();
   init_metadata();
-  init_geometry(material_overrides, temperature_overrides);
+  init_geometry(material_overrides, temperature_overrides, density_overrides);
 }
 
 void DAGUniverse::set_id()
@@ -179,7 +184,8 @@ void DAGUniverse::initialize()
 }
 
 void DAGUniverse::initialize(const MaterialOverrides& material_overrides,
-  const TemperatureOverrides& temperature_overrides)
+  const TemperatureOverrides& temperature_overrides,
+  const DensityOverrides& density_overrides)
 {
 #ifdef OPENMC_UWUW_ENABLED
   // read uwuw materials from the .h5m file if present
@@ -190,7 +196,7 @@ void DAGUniverse::initialize(const MaterialOverrides& material_overrides,
 
   init_metadata();
 
-  init_geometry(material_overrides, temperature_overrides);
+  init_geometry(material_overrides, temperature_overrides, density_overrides);
 }
 
 void DAGUniverse::init_dagmc()
@@ -227,7 +233,8 @@ void DAGUniverse::init_metadata()
 }
 
 void DAGUniverse::init_geometry(const MaterialOverrides& material_overrides,
-  const TemperatureOverrides& temperature_overrides)
+  const TemperatureOverrides& temperature_overrides,
+  const DensityOverrides& density_overrides)
 {
   moab::ErrorCode rval;
 
@@ -324,6 +331,31 @@ void DAGUniverse::init_geometry(const MaterialOverrides& material_overrides,
       }
     }
 
+    if (density_overrides.count(c->id_)) {
+      if (c->material_.empty() || c->material_[0] == MATERIAL_VOID) {
+        fatal_error(fmt::format("DAGMC cell {} was specified with a density "
+                                "but no non-void material.",
+          c->id_));
+      }
+      // density_mult_ holds the true density until materials are finalized,
+      // at which point it is converted to a proper multiplier (same as CSG).
+      c->density_mult_ = density_overrides.at(c->id_);
+
+      if (settings::verbosity >= 10) {
+        const auto& dens = density_overrides.at(c->id_);
+        std::stringstream override_values;
+        for (size_t i = 0; i < dens.size(); ++i) {
+          if (i > 0)
+            override_values << " ";
+          override_values << dens[i];
+        }
+        write_message(fmt::format("Overriding DAGMC cell {} property "
+                                  "'density [g/cm³]' with value(s): {}",
+                        c->id_, override_values.str()),
+          10);
+      }
+    }
+
     // check for temperature assignment
     std::string temp_value;