Adding some documentation about how to configure and install with DAGMC.

docs/source/io_formats/settings.rst

@@ -88,6 +88,14 @@ you care. This element has the following attributes/sub-elements:
 
     *Default*: 0.0
 
+--------------------------------
+``<dagmc>`` Element
+--------------------------------
+
+When the DAGMC mode is enabled, the OpenMC geometry will be read from the file
+``dagmc.h5m``. If a `geometry.xml <../io_formats/geometry.html>`_ file if
+present, it will be ignored.
+    
 --------------------------------
 ``<electron_treatment>`` Element
 --------------------------------

docs/source/usersguide/geometry.rst

@@ -398,3 +398,22 @@ if needed, lattices, the last step is to create an instance of
 
 .. _constructive solid geometry: http://en.wikipedia.org/wiki/Constructive_solid_geometry
 .. _quadratic surfaces: http://en.wikipedia.org/wiki/Quadric
+
+--------------------------
+Using a DAGMC CAD Model
+--------------------------
+
+A DAGMC run can be enabled in OpenMC by setting the ``dagmc`` property to
+``True`` in the model settings either via the Python ``openmc.settings``
+module::
+
+  settings = openmc.Settings()
+  settings.dagmc = True
+
+or in the `settings.xml <../io_formats/settings.html>`_ file::
+
+  <dagmc>true</dagmc>
+
+With ``dagmc`` set to true, OpenMC will load the DAGMC model (named
+``dagmc.h5m``) when initializing a simulation. If a `geometry.xml
+<../io_formats/geometry.html>`_ is present, it will be ignored.

docs/source/usersguide/install.rst

@@ -159,14 +159,25 @@ Prerequisites
           sudo apt install mpich libmpich-dev
           sudo apt install openmpi-bin libopenmpi-dev
 
+    * DAGMC_ toolkit for simulation using CAD-based geometries
+
+      OpenMC supports particle tracking in CAD-based geometries via the Direct
+      Accelerated Geometry Monte Carlo (DAGMC) toolkit (`installation
+      instructions
+      <https://svalinn.github.io/DAGMC/install/dag_multiple.html>`_). For use in
+      OpenMC, only the ``MOAB_DIR`` and ``BUILD_TALLY`` variables need to be
+      specified in the CMake configuration step.
+    |
     * git_ version control software for obtaining source code
 
+
 .. _gfortran: http://gcc.gnu.org/wiki/GFortran
 .. _gcc: https://gcc.gnu.org/
 .. _CMake: http://www.cmake.org
 .. _OpenMPI: http://www.open-mpi.org
 .. _MPICH: http://www.mpich.org
 .. _HDF5: https://www.hdfgroup.org/solutions/hdf5/
+.. _DAGMC: https://svalinn.github.io/DAGMC/index.html
 
 Obtaining the Source
 --------------------