# 4.4.1.3. MPMD Radiation

This chapter outlines the use of Multiple-Program-Multiple-Data (MPMD) coupling to perform radiation solves with Sierra/PMR. MPMD coupling allows the radiation solve to occur on an entirely different set of processors using an executable other than Aria. See PMR - Radiation for a complete example of an MPMD radiation problem.

For information on launching an MPMD simulation, see Running MPMD Jobs. Note that for the case of the Aria-PMR coupling, Aria must be the `mpmd_master`, so an example launch line would be

    $ mpirun -np 100 aria --mpmd_master --mpi_color 9999 \
        -i input.i : -np 100 pmr -i pmr.i

## 4.4.1.3.1. Aria Input Deck

Enabling MPMD radiation coupling in the Aria input deck requires the addition of an MPMD Coupling block to the Aria region. See the command reference for details on all available lines.

    # Scope: Sierra > Procedure > Aria Region
    Begin MPMD Coupling
      Coupling Type is Radiation
      Begin Radiation Coupling
        Iterations per radiation solve is #
        #    -or-
        Time steps per radiation solve is #
      End Radiation Coupling
    End MPMD Coupling

The only option to specify is the frequency with which the radiation transport equation solve should occur with respect to the Aria equation solves. Specifying Iterations per radiation solve will set the RTE to be solved every specified number of Aria non-linear iterations. Specifying Time steps per radiation solve will set the RTE to be solved every specified number of time steps. Both options cannot be specified simultaneously and neither are required. The default behavior is to run the RTE solve every iteration.

The mesh parts that the RTE should be solved on are specified by including them as equation descriptions lines in the input file as described in Equation Specification.

## 4.4.1.3.2. PMR Input Deck

The PMR input deck syntax is described in the current PMR user manual. It is YAML formatted and can be quite terse for most cases. The common parameters to set in the PMR input file are the mesh file to use, the quadrature order, solution method, and linear solver settings. The RTE equation does not have a time term, so no transient information is needed for PMR. The PMR code will terminate automatically when Aria completes its execution.