Keywords: developer_man

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# 4 Parallelization

The `PHonon` package uses the same parallelization mechanisms of the QUANTUM ESPRESSO package. See the Developer manual in the `Doc` directory two levels above this one for more information. It is parallelized on plane-waves, pools, bands, and images. The `-ortho` flag is not used. `Scalapack` routines are not used in the `ph.x` code.

Each tensor should be collected as soon as it is calculated and all processors must have the same tensors. Please avoid to collect tensors in routines distant from where they are calculated. There might be exception to this rule for efficiency, but please try not to abuse for small arrays. Only collected quantities are saved on the `.xml` file, so they should not depend on the parallelization level. Note that only ionode writes the `.xml` files, so you have different `xml` files only for different images. The variables are then broadcasted to all processors in an image.