2. Install LAMMPS

You can download LAMMPS as an executable or as source code.

With source code, you also have to build LAMMPS. But you have more flexibility as to what features to include or exclude in the build. If you plan to modify or extend LAMMPS, then you need the source code.

These are the files and sub-directories in the LAMMPS distribution:

README text file
LICENSE GNU General Public License (GPL)
bench benchmark problems
cmake CMake build files
doc documentation
examples simple test problems
lib additional provided or external libraries
potentials interatomic potential files
python Python wrapper on LAMMPS
src source files
tools pre- and post-processing tools

You will have all of these if you download source. You will only have some of them if you download executables, as explained on the pages listed above.