small cleanup

LAMMPS is written to use the ubiquitous `MPI (Message Passing Interface)

<https://en.wikipedia.org/wiki/Message_Passing_Interface>`_ library API

for distributed memory parallel computation.  It is compatible with the

MPI standard version 2.x and later.  LAMMPS can also be build into a

for distributed memory parallel computation.  You need to have such a

library installed for building and running LAMMPS in parallel using a

domain decomposition parallelization.  It is compatible with the MPI

standard version 2.x and later.  LAMMPS can also be built into a

"serial" executable for use with a single processor using the bundled

MPI STUBS library.

Independent of the distributed memory MPI parallelization parts of

Independent of the distributed memory MPI parallelization, parts of

LAMMPS are also written with support for shared memory parallelization

using the OpenMP threading standard. A more detailed discussion of that

is below.