The deMon2k code is parallelized using the MPI (Message Passing Interface) routines from the 3.0 standard. The MPI routines are addressed in the deMon2k source by interface subroutines. If you want to run deMon2k in parallel, you need to install the libraries for the MPI 3.0 standard in your system first (they are not included in the deMon2k package).