The DAG-MCNP input file
~~~~~~~~~~~~~~~~~~~~~~~

Note that all the information here applies to both DAG-MCNP5 and DAG-MCNP6.

The DAG-MCNP input file contains only the data cards section of a standard MCNP
input file. There are no cell or surface cards included in the input file.

A new data card has been added to DAG-MCNP to define parameters for the DAGMC
geometry capability.
::

  Form: dagmc  keyword1=value   keyword2=value
       check_src_cell: behavior of CEL variable in SDEF card
                          on  [default] standard interpretation for
                                        CEL variable: source rejection
                          off           no cell rejection - assume that
                                        sampled position is in cell CEL
       overlap_thickness: allows particle tracking through small overlaps
                          {real} [default=0.0]
               distlimit: toggle usage of flight distance sampled from
                          physics to accelerate ray-tracing search
                          off [default] do not use physics flight distance
                          on            do use physics flight distance

Running DAG-MCNP
~~~~~~~~~~~~~~~~

Running DAG-MCNP is identical to running the standard MCNP, but a few new
keywords have been added to the command line to specify the necessary files.

:``gcad=<geom_file>`` (required): Specify the filename of the input geometry
    file. It can be in one of two formats: the MOAB (\*.h5m) format (this is the
    format produced by ``export dagmc`` in Cubit), or a facet file
    produced by DAGMC. If this entry is not present, DAG-MCNP will assume that
    it is running in native MCNP mode.

:``fcad=<facet_file>`` (optional): [default: fcad] Specify the filename of the
    output facet file. This is the file produced by DAGMC that contains the
    geometry as well as the products of a number of preprocessing steps, which
    can be quite time-consuming. This file can be used as input with the
    ``gcad=`` keyword in subsequent runs to avoid spending time redoing the
    preprocessing steps.

:``lcad=<log_file>`` (optional): [default: lcad] Specify the filename of the
    output log file. This is a skeleton of an MCNP file which contains
    information about the volumes, surfaces, materials and material assignments,
    boundary conditions, and tallies defined in the geometry. If you specify a
    name other than the default for this file on the command-line, that file
    will be used instead of the one generated automatically by DAG-MCNP. This
    can be useful if you want to make small changes to your material
    assignments, importances, etc., but it cannot be used to change anything
    about the geometry. It is up to the user to ensure that the geometry file
    and log file being used correspond to each other. This runtime parameter is
    unique to DAG-MCNP.

To run DAG-MCNP, you must make the minimum input deck that defines the particle
source definition, runtime parameters, and physics cutoffs. You then run
DAG-MCNP5 with
::

    $ mcnp5 i=input g=geom.h5m

If DAG-MCNP is unable to find some nuclides, you can modify the lcad file that
was produced in the previous step to remove extraneous nuclides, and re-run with
::

    $ mcnp5 i=input g=geom.h5m l=lcad_modified
