DAGMC H5M Files

The primary output format. Creates triangular surface meshes for use with DAGMC-enabled transport codes (OpenMC, MCNP, FLUKA, etc.).

Basic Export

import cadquery as cq
from cad_to_dagmc import CadToDagmc

assembly = cq.Assembly()
assembly.add(cq.Workplane("XY").sphere(10), name="sphere")

model = CadToDagmc()
model.add_cadquery_object(assembly, material_tags=["tungsten"])

model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    min_mesh_size=0.5,
    max_mesh_size=10.0,
)

With Different Backends

Choose between h5py (default) and pymoab backends:

import cadquery as cq
from cad_to_dagmc import CadToDagmc

sphere1 = cq.Workplane().sphere(5)
sphere2 = cq.Workplane().moveTo(10, 0).sphere(2)

assembly = cq.Assembly()
assembly.add(sphere1)
assembly.add(sphere2)

model = CadToDagmc()
model.add_cadquery_object(cadquery_object=assembly, material_tags=["mat1", "mat2"])

# Export using h5py backend (default, no MOAB needed)
model.export_dagmc_h5m_file(
    filename="dagmc_h5py.h5m",
    h5m_backend="h5py",
)

# Export using pymoab backend (requires MOAB installation)
model.export_dagmc_h5m_file(
    filename="dagmc_pymoab.h5m",
    h5m_backend="pymoab",
)

With Meshing Backends

Choose between the cad-to-dagmc-mesher (default) and GMSH meshing backends:

# cad-to-dagmc-mesher backend (default) - surface and volume meshing
model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    meshing_backend="cad-to-dagmc-mesher",
    tolerance=0.5,
    angular_tolerance=0.2,
)

# GMSH backend - full control over mesh parameters
model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    meshing_backend="gmsh",
    min_mesh_size=0.5,
    max_mesh_size=10.0,
    mesh_algorithm=1,
)

API Reference

export_dagmc_h5m_file()

Common Parameters:

Parameter	Type	Default	Description
filename	str	“dagmc.h5m”	Output file path
scale_factor	float	1.0	Geometry scale factor. See the note below on how it affects the units of the linear mesh sizing parameters
imprint	"partial", "full", None, or False	"partial"	Shared-surface imprint mode. None still imprints, with glue disabled; only False skips imprinting. See Imprinting for options.
imprint_threads	"all" or positive int	"all"	Keyword-only thread count for imprinting. See Imprinting for validation and scope.
mesh_threads	"all" or positive int	"all"	Keyword-only thread count for cad-to-dagmc-mesher meshing after the imprint. Fewer threads lower peak RAM; the mesh is the same. The gmsh backend warns that it is unused. See Parallel Processing.
implicit_complement_material_tag	str	None	Void space material tag

Backend Selection:

Parameter	Type	Default	Description
meshing_backend	str	auto	"cad-to-dagmc-mesher" or "gmsh". Auto-selected from the other arguments provided. Defaults to "cad-to-dagmc-mesher" when no backend-specific arguments are given, falling back to "gmsh" if cad-to-dagmc-mesher is not installed.
h5m_backend	str	“h5py”	"h5py" or "pymoab" for writing h5m files

GMSH Backend Parameters:

These parameters only apply when using meshing_backend="gmsh" (or when auto-selected):

Parameter	Type	Default	Description
min_mesh_size	float	None	Minimum mesh element size
max_mesh_size	float	None	Maximum mesh element size
mesh_algorithm	int	1	GMSH 2D surface meshing algorithm
mesh_algorithm_3d	int	1	GMSH 3D volume algorithm: 1 = Delaunay, 10 = HXT
volume_mesh_options	dict	None	Numeric/string GMSH options applied after surface meshing, before volume meshing; only used with unstructured_volumes
method	str	“file”	CAD transfer method: "file" or "in memory"
set_size	dict	None	Per-volume mesh sizes. Keys can be volume IDs (int) or material tag names (str).
unstructured_volumes	list	None	Volume IDs (int) or material tags (str) for conformal volume mesh
umesh_filename	str	“umesh.vtk”	Output filename for unstructured volume mesh

mesh_algorithm_3d and volume_mesh_options configure the volume step without changing how the DAGMC surfaces are initially meshed. See 3D algorithms and volume-only options for HXT and relaxed interior sizing. The existing surface mesh is reused for the unstructured output.

cad-to-dagmc-mesher Backend Parameters:

These parameters only apply when using meshing_backend="cad-to-dagmc-mesher":

Parameter	Type	Default	Description
tolerance	float	0.5	Linear tolerance for the surface mesh, in cm after scale_factor
angular_tolerance	float	0.2	Angular tolerance for the surface mesh
tet_volumes	Iterable[str]	None	Material tag names of the volumes to fill with tetrahedra
target_edge_length	float	None	Target tetrahedron edge length, in scaled-geometry units
umesh_filename	str	“umesh.vtk”	Output filename for unstructured volume mesh

Important

scale_factor and the units of linear mesh sizes. All the linear mesh sizing parameters are in the units of the scaled geometry, so the same number means the same deflection on the output mesh whichever backend you use:

Backend	Linear sizing parameters
gmsh	min_mesh_size, max_mesh_size, set_size
cad-to-dagmc-mesher	tolerance, target_edge_length

OpenMC works in centimetres, so set scale_factor to bring the CAD to cm (for example scale_factor=100 for a model in metres). The cad-to-dagmc-mesher default tolerance of 0.5 is then a 5 mm deflection.

angular_tolerance is an angle and is unaffected by scale_factor.

tet_volumes and target_edge_length must be given together to write a volume mesh.

Warning

Do not mix GMSH and cad-to-dagmc-mesher backend parameters in the same call. If you provide parameters from both backends without explicitly setting meshing_backend, an error will be raised.

Returns:

• str - Path to the created h5m file

• Or tuple[str, str] - (h5m_path, vtk_path) when a volume mesh is also written, which is when unstructured_volumes is set on the GMSH backend, or when tet_volumes and target_edge_length are set on the cad-to-dagmc-mesher backend

Using in OpenMC

Load the DAGMC geometry in OpenMC:

import openmc

# Define materials (names must match material tags)
tungsten = openmc.Material(name="tungsten")
tungsten.add_element("W", 1.0)
tungsten.set_density("g/cm3", 19.3)

steel = openmc.Material(name="steel")
steel.add_element("Fe", 1.0)
steel.set_density("g/cm3", 7.8)

materials = openmc.Materials([tungsten, steel])

# Load the DAGMC geometry
dag_universe = openmc.DAGMCUniverse(filename="dagmc.h5m")
geometry = openmc.Geometry(root=dag_universe.bounded_universe())

# Set up settings
settings = openmc.Settings()
settings.batches = 10
settings.particles = 1000
settings.run_mode = "fixed source"

model = openmc.Model(geometry=geometry, materials=materials, settings=settings)
model.run()

See Also

• H5M Backends - h5py vs pymoab comparison

• GMSH Backend - GMSH meshing options

• cad-to-dagmc-mesher Backend - cad-to-dagmc-mesher meshing options

• Conformal Meshes - Combined surface and volume output