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Yes, in fact you have to. NWChem is inherently parallelized and designed to scale well on a multi-processor machine or a cluster. The underlying parallel system is MPI (Message Passing Interface) which is a commonly available standard that runs on many platforms. Consult our Parallel Programing FAQ and follow some of the links in there if you want to have more information about MPI. Even if you want to use only one processor for your NWChem run (which sometimes is the best solution, particularly for smaller computations), you have to submit the program to a parallel environment. In the case of our SunFire cluster, this environment is called CRE (Cluster RuntimeEnvironmment), and the relevant command is mprun:
mprun -np 8 nwchem sample > sample.log
This will run your sample.nw input file on eight processors. If you want to run the program in serial, you have to use the '-np 1' option in the above command line. Note that you are only allowed to run NWChem this way for small test systems! For any production jobs, you have to submit the task to the GridEngine queuing software (see next section.).