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## 3.2 Running on parallel machines

Parallel execution is strongly system- and installation-dependent. Typically one has to specify:

1. a launcher program such as `mpirun` or `mpiexec`, with the appropriate options (if any);
2. the number of processors, typically as an option to the launcher program;
3. the program to be executed, with the proper path if needed;
4. 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.