how to run CFDEM tutorial cases in serial

hi, all-

Sorry if this is a stupid question. I've run tutorial cases like ErgunTestMPI in cfdemSolverPiso and twoSpheresGlowinskiMPI in cfdemSolverIB. Both of them can be run in parallel (like 4 cores or 2 cores) with no problem.

However, when I tried to run them in only one core (in serial), I got errors as below:

--> FOAM FATAL ERROR:
bool IPstream::init(int& argc, char**& argv) : attempt to run parallel on 1 processor

From function UPstream::init(int& argc, char**& argv)
in file UPstream.C at line 79.

FOAM aborting

What I do on the tutorials is just to modify the corresponding setup for number of processors in decomposeParDict, in.liggghts_run and parCFDDEMrun.sh. Why that works when I change processor number from 4 to 2 but not when from 4 to 1? Did I miss anything or these tutorials cannot run on one core at all?

Any suggestion will be appreciated. Thank you in advance!

Zhuang