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Parallel execution is strongly system- and installation-dependent.
Typically one has to specify:
- a launcher program such as mpirun or mpiexec,
  with the  appropriate options (if any);
- the number of processors, typically as an option to the launcher
  program;
- the program to be executed, with the proper path if needed;
- other QUANTUM ESPRESSO-specific parallelization options, to be
  read and interpreted by the running code.
Items 1) and 2) are machine- and installation-dependent, and may be
different for interactive and batch execution. Note that large
parallel machines are  often configured so as to disallow interactive
execution: if in doubt, ask your system administrator.
Item 3) also depend on your specific configuration (shell, execution path, etc).
Item 4) is optional but it is very important
for good performances. We refer to the next
section for a description of the various
possibilities.