Installing OpenMPI on Apple macOS

Here’s some brief notes to install OpenMPI on a current (2020) Apple system running macOS Catalina 10.15.

Install Apple Xcode. You can download Xcode at https://developer.apple.com/xcode/.
Install Xcode command line tools. Input the following command string in Terminal:
xcode-select --install
Install the Intel C and Fortran compilers, including the Math Kernel Library. After you install them, make sure you add the line source /opt/intel/bin/compilervars.sh intel64 to your .profile or .bash_profile file in your home directory so that the terminal will know where the compilers are located at. See the Intel compiler installation help website for more details on that.
Download and install Open-MPI. You can obtain Open-MPI source code from https://www.open-mpi.org/software/ompi/v4.0/. Unpack or unzip the files. Open Terminal.app and cd to the open-mpi folder. Run the configure script by pasting the following into the terminal (this should be one line of text):
```
sudo  ./configure --prefix=/opt/openmpi CC=icc CXX=icc F77=ifort FC=ifort \
lt_prog_compiler_wl_FC='-Wl,';
make all install
```
Then go do something else for the annoying hour or more that it takes to build MPI. When this is done, you will have the mpi compilers and libraries installed into /opt/openmpi.

Warning

If you update to a new version of macOS or a new version of Xcode, you will likely have to reinstall the Intel compilers and rebuild OpenMPI, as the compilers depend on libraries contained in Xcode. I strongly recommend moving any previously installed version of OpenMPI to a different folder so that you get a clean build (i.e. run sudo mv /opt/openmpi /opt/openmpi_old before builing the new openmpi), as in the past there have been file conflict issues from lingering deprecated files in the openmpi library.