What commands were used to build the charmm program? Were there any informational or warning messages regarding MPI from the build process?
The MPI libraries from Linux distributions typically lack the Fortran90 bindings (mpi.mod). With the GCC suite (gcc, gfortran, etc.), it is recommended to build the MPI library with Fortran90 support included, using the same compiler versions that will be used to compile charmm.
Please note that there is a specific forum for installation questions.