How to resolve "mpirun: command not found" error on Mac I'm running scientific simulation software (lammps) and whenever I try to submit the job by using a shell script which contains...

mpirun -np 2 /Users/mustafaaslamy/Downloads/lammps-16Mar18/src/lmp_serial -in in.airebo

...it throws the error of mpirun:command not found. 

How do I resolve this?

I'm using a MacBook Air (early 2014), processor 1.4 GHz Intel Core i5, software OS X 10.9.5 (13F34).
You are trying to use a program but the Mac does not find the program. It's probably because it's not installed. The program you are trying to use is called Open MPI, here is how to install it: https://intothewave.wordpress.com/2011/12/27/install-open-mpi-on-mac-os-x/

After installing it you will be able to use the mpirun command. Because Open MPI isn't an Apple product, I recommend you to go to stackoverflow.com if you have questions regarding Open MPI.