Example: Athermal, gas phase, reversible reaction with UDF
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. math::

  A(g) ↔ R(g)

Notes:
 * Species database names and alias are defined on the same line.
 * The fluid cell index (``IJK``) is passed as a dummy argument.
 * Global field variables are referenced (RO_g, X_g, T_g, and EP_g )

.. code-block:: none

    ### mfix.dat:
    NMAX_g = 2                     ! No. of gas phase species
    Species_g(1) = "A" "R"         ! Database names
    Species_Alias_g(1) = "A" "R"   ! Species Aliases

    @(RXNS)             !  Begin reaction block
    fwd_AtoR {          !  Reaction 1 construct
    chem_eq = "A --> R" !  Chemical Reaction Eq
    DH = 0.0            !  (cal/moles-reacted)
    fracDH(0) = 1.0     !  Gas phase HoR
    }                   !  End reaction 1 construct
    rvs_AtoR {          !  Reaction 2 construct
    chem_eq = "R --> A" !  Chemical Reaction Eq
    DH = 0.0            !  (cal/moles-reacted)
    fracDH(0) = 1.0     !  Gas phase HoR
    }                   !  End reaction 2 construct
    @(END)              !  End reaction block

.. code-block:: fortran

    usr_rates.f:
    SUBROUTINE USR_RATES(IJK, RATES)

    INCLUDE 'species.inc'

    INTEGER, INTENT(IN) :: IJK
    DOUBLE PRECISION, DIMENSION(NO_OF_RXNS), INTENT(OUT) :: RATES


    ! Fluid Cell Index
    DOUBLE PRECISION c_A ! species A concentration (mol/cm^3)
    DOUBLE PRECISION c_R ! species R concentration (mol/cm^3)

    ! Calculate species concentrations:
    c_A = (RO_g(IJK) * X_g(IJK,A))/MW_g(A)
    c_R = (RO_g(IJK) * X_g(IJK,R))/MW_g(R)

    ! Forward Reaction: A --> R (reacted moles/sec.cm^3)
    !-------------------------------------------------------//
    RATES(fwd_AtoR) = 1.2d17 * exp(-5.837d3/T_g(IJK)) * &
    EP_g(IJK) * c_A

    ! Reverse Reaction: R --> A (reacted moles/sec.cm^3)
    !-------------------------------------------------------//
    RATES(rvs_AtoR) = 2.5d41 * exp(-1.4897d4/T_g(IJK)) * &
    EP_g(IJK) * c_R

    END SUBROUTINE USR_RATES