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<H2><A ID="SECTION00052000000000000000">
4.2 Parallelization levels</A>
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<P>
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:
<PRE>
world - image_group - PWscf hierarchy
</PRE>
<P>
<TT>world</TT>: is the group of all processors (MPI_COMM_WORLD).
<P>
<TT>image_group</TT>: Processors can then be divided into different image groups,
each of them taking care of one or more NEB images.
<P>
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 <TT>PWscf</TT>).
<P>
The default number of image groups is one, corresponding to the option
<TT>-ni 1</TT> (or, equivalently, <TT>-nimage 1</TT>).
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