Hi, so you need to have an MPI library implementation on your computer. For example, you could use OpenMPI, MPICH, or the Intel MPI implementation (or some others).
Then you need to include the mpi.h header when you compile, by including a flag like -I/path/to/mpi/headers
For example, on my Mac I installed OpenMPI using Homebrew. So I included the flags:
in my compiler flags (for my driver application, not for ITensor itself).
Also you need to include "linker" flags to link to the MPI library itself. Again, on my Mac I include the flags:
-L/usr/local/Cellar/open-mpi/1.10.1/lib -lmpi_cxx -lmpi
A good way to find out the compiler flags for your particular MPI implementation is to run the program mpicxx which is often included with many MPI implementations (it is a special compiler command that automatically includes the MPI flags). You can either use mpicxx itself as your compiler (i.e. in place of a command like gcc or clang++). Or mpicxx often includes an option called "showme" that will show you the best compiler flags to use: