It should work, but there may a number of variables influencing the behavior, such as the compiler suite and version, the MPI implementation and version, other actions being done in the CHARMM input file, and the incomplete implementation of the file format by the NAMD/VMD developers. (They don't properly use all of the data fields in the header record, and that has caused no end of problems in trying to read DCD files produced by other programs into CHARMM, esp. those mangled by the VMD tools.)

If you only wish to use a single core, you could try running the prog w/o using mpirun at startup. You probably need to do the MPI setup, though, due to runtime library dependencies.

Also, it could be a bug, so sometimes using a different CHARMM version can resolve these issues; c45b1 was released in August 2020.

Rick Venable
computational chemist