You can use OpenMPI and MPICH from MacPorts for both gfortran and nagfor
by setting various environment variables and/or changing the scripts.

To install OpenMPI from MacPorts on OS 15.7.4 or later:
<pre>
  sudo -s
  port install openmpi-gcc15
  # If this is the only MPI library to be used, make it default:
  port select --set mpi openmpi-gcc15
</pre>
To install MPICH from MacPorts on OS 15.7.4 or later:
<pre>
  sudo -s
  port install mpich-gcc15
  # If this is the only MPI library to be used, make it default:
  port select --set mpi mpich-gcc15
  # Fix the mpifort script to work for nagfor too: 
  cd /usr/local/bin/
  cp mpifort-mpich-gcc15 mpifort-mpich-gcc15.orig
  # set  final_ldflags=" " and extra_f77_flags=" "
  emacs mpifort-mpich-gcc15
</pre>
In addition, use "setenv MPICH_FC nagfor" or "export MPICH_FC=nagfor" when using nagfor
instead of the default gfortran.

Alternatively, you can use the precompiled MPICH libraries configured for the nagfor compiler on OS systems.

The following MPI library works for Macintosh system using ARM chips
<p>
  <A HREF=mpich-4.1.2_nagfor_arm.tgz>mpich-4.1.2_nagfor_arm.tgz</A> for OS 15.7.3 or before<br>
  <A HREF=mpich-5.0.0_nagfor_arm.tgz>mpich-5.0.0_nagfor_arm.tgz</A> for OS 15.7.4 or newer
<p>
As root (sudo -s), open the tar file inside NAG_fortran and link it
<pre>
cd /usr/local
rm -rf mpich lib/NAG_Fortran/mpich
tar xzf ~/Downloads/mpich*_nagfor_arm.tgz
ln -s mpich lib/NAG_Fortran/mpich  # this may or may not be needed
</pre>
The following MPI library only works for 64-bit Macintosh system using Intel chips:
<p>
<A HREF=mpich-3.2.1_nagfor_fpp.tgz>mpich-3.2.1_nagfor_fpp.tgz</A>
<p>
As root (sudo -s), open the tar file and link it
<pre>
cd /usr/local/
tar xzf mpich-3.2.1_nagfor_fpp.tgz
rm -f mpich
ln -s mpich-3.2.1 mpich
</pre>
Exit from the super user mode (Ctrl-D or exit).
<p>
In both cases include /usr/local/mpich/bin into the path in the ~/.cshrc
or similar file, then
<pre>
source .cshrc
rehash
</pre>
Then type
<pre>
which mpif90
</pre>
to see if it is found. Now you should be able to compile the code
with NAG Fortran and MPICH. To avoid run time errors due to some security
settings (software is not from Apple), it may be necessary to do the
following:
<pre>
xattr -d com.apple.quarantine /usr/local/mpi/bin/mpiexec
xattr -d com.apple.quarantine /usr/local/mpi/bin/hydra_pmi_prox
spctl --master-disable
</pre>
Possibly the last command by itself is sufficient.
<p>
For some MPI implementations it may be necessary to create
a public SSH key of the machine with
<pre>
ssh-keygen 
</pre>
(hit return to all questions) and the resulting public key
file should be added to the authorized keys:
<pre>
cat .ssh/id_rsa.pub >> .ssh/authorized_keys
</pre>
Even if not needed for running MPI parallel code, the public key is useful 
for passwordless ssh/scp/rsync etc. access.