Keywords: user_guide

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## 4.2 Parallelization levels

Data structures are distributed across processors. Processors are organized in a hierarchy of groups, which are identified by different MPI communicators level. The groups hierarchy is as follow:

      world - image_group - PWscf hierarchy

`world`: is the group of all processors (MPI_COMM_WORLD).

`image_group`: Processors can then be divided into different image groups, each of them taking care of one or more NEB images.

Image parallelization is of loosely coupled type, so that processors belonging to different image groups communicate only once in a while, whereas processors within the same image group are tightly coupled and communications are more significant (please refer to the user guide of `PWscf`).

The default number of image groups is one, corresponding to the option `-ni 1` (or, equivalently, `-nimage 1`).